If you aspire to become a great manager, simply striving to meet your company's actual (versus espoused) standards probably won't get you there.

By Linda A. Hill and Kent Lineback, contributors

FORTUNE -- One morning, years ago, Kent heard loud laughter outside his office and found several people clustered around Charley, the head graphic designer who was easily the largest and most popular member of the marketing department Kent ran.

"Charley lost 25 pounds," someone said when Kent joined the group. Charley's weight was a common topic of office conversation but only because Charley himself brought it up so often.

"That's great, Charley," Kent said. "Congratulations!" Then -- always the boss, though this was hardly a management matter -- he asked, "What's your goal?"

The smile dropped from Charley's face.

"That," he said, "was it."

It wasn't one of Kent's most sensitive moments. Later, when he apologized to Charley one-on-one, Charley said, "You were right. I need to lose more. But I got in a group and we all swore do-or-die to lose 25 pounds. That became my goal."

To be sure, there's little connection between losing weight and business performance except that Charley's experience illustrates a principle -- the Opportunity Gap -- that we in business often overlook because we work in groups and group norms are powerful forces of influence.

See also: Dating and business: Not all that different

An opportunity gap is the difference between our current performance and what we're capable of doing. Many times, though, we focus not on our opportunity gap but on our performance gap, the difference between current performance and what we're expected to do. We're content to achieve what others expect rather than what we're truly capable of doing.

Yes, expectations can sometimes exceed our capabilities. A medical condition may have made weight loss extremely difficult for Charley. Or a salesman may be given a quota that truly is unrealistic. But expectations often do reflect only a portion of what's possible. Charley, for example, ultimately shed four times what he lost in his weight loss group. That was his opportunity gap. But, for a time, he fell into the trap of accepting his performance gap, the 25 pounds expected by the group, as his ultimate goal.

We believe the disparity between performance and opportunity gaps goes far to explain something we've observed in our combined 60 years of teaching, researching, practicing, and observation: most managers fall short of what they could be. They simply stop getting better at what they do.

We see this every time we ask bosses to assess themselves as leaders and managers. The great majority say they could be better. When we ask how they could be better, many can identify at least one area -- "delegation" is a common answer -- where they have room for improvement.

If many, and perhaps most, managers think they could and even should be better, why don't they take active steps to improve?

Becoming a great manager requires persistent effort over a long period of time. It can require difficult personal change. Converting management principles into practice isn't easy. There aren't many role models.

All those reasons play their roles, but we think one is key: Companies simply don't expect enough of their managers. A statistic we came across recently astounded us: A 2009 survey of companies by Bersin and Associates revealed that they considered nearly four of every 10 mid-level managers no better than "poor" or "fair."

See also: Internal competition at work: Worth the trouble?

This figure is an admission by companies that they are failing in their fundamental task of creating a corps of competent, capable managers. If 40% of their mid-level -- not first-level -- managers are mediocre at best, why don't they recognize that they face a corporate crisis and take dramatic steps to improve?

We're not aware of any companies that have declared such a state of emergency, which seems to confirm something our experience and observations have also told us: even though companies may claim, implicitly or explicitly, that they set high standards for their managers, they in fact condone managerial practices and behavior that persistently fall short of those standards.

This whole topic deserves more study and discussion, but for individual managers, it seems to convey one clear point: if you aspire to become a great manager, simply striving to meet your organization's actual, versus espoused, standards probably won't get you there. Being rated "meets expectations" or even better isn't necessarily a good measure of your skill or progress.

To grow as a manager, you will need to define your own opportunity gap – identify what you can become -- rather than accept the actual performance gap for managers in your firm. This is the first step in taking responsibility for your own development.

Note: We adapted the terms "Opportunity Gap" and "Performance Gap" from Winning Through Innovation: A Practical Guide to Leading Organizational Change and Renewal by Michael L. Tushman and Charles A. O'Reilly II.