Today, on National Equal Pay Day, it’s worth noting that women still make only 77 cents vs. the average man’s dollar.

I may catch flak for saying this, but one reason is that women aren’t as good at negotiating as men are. I know this from talking with hundreds of women in the Fortune‘s Most Powerful Women community. And many women on our annual Most Powerful Women list have told me that they’re not nearly as comfortable promoting themselves as guys are.

I couldn’t help thinking about this yesterday at the Matrix Awards, where some 1,200 people packed the ballroom at New York’s Waldorf-Astoria to toast the most extraordinary women in media.

Even as they were celebrated, some honorees downplayed their power or observed how freely it did not flow.

Rosie O’Donnell joked that the Matrix organizers crimped her freedom to speak as outrageously as she likes to do. Her publicist Cindi Berger, a Matrix winner, made her dress in “a bejeweled tunic” for the first time in her life, Rosie said.

PBS political journalist Gwen Ifill found out early on that she couldn’t be “superwoman.” she told the Matrix crowd. “Like most women, I just get up in the morning determined to do the best I can each day for my family, my profession, and my world. It comes as a mild shock whenever I am recognized for my efforts.”

Sheryl Sandberg, another 2011 Matrix honoree, has achieved a lot. At 41, she is COO of Facebook and No. 16 on the Fortune MPWomen list. Yet she says,”Certainly at times I have not raised my hand or felt confident.”

It drives Sandberg crazy that young women at Facebook and at Google , where she previously worked, put brakes on their career advancement as they contemplate having kids. Sandberg’s argument against leaning back in your career: “Don’t Leave Before you Leave,” an essay she wrote for Fortune in 2009 about going for the big job, no matter. Among young women in Silicon Valley, Sandberg’s essay has become a must-read.

Coincidentally, I ran into Sandberg last night at a restaurant in mid-town Manhattan–she was having dinner with Arianna Huffington, who recently sold the Huffington Post to AOL and yesterday presented her Matrix award to her. Standing outside Milos Estiatorio on a summer-like night, Sandberg and I talked about our mutual reluctance to put ourselves out there as role models, much less to swagger.

You can see the entire Matrix Awards on this Facebook page, appropriately. One once-powerful woman in the media universe was conspicuously absent from the fete: Cathie Black. Take a look at the reader comments below my Friday Postcard–where I talked exclusively to the former Hearst Magazines chairman about her ouster from her job as chancellor of New York City’s public schools. Those comments will give you a sense of the flak that hits many women who chase, or simply accept, power beyond the socially acceptable norms.

Black didn’t have experience in education, and she probably shouldn’t have accepted the NYC schools job. (Though she told me on Friday that she has “no regrets” that she did.) But the fact is, the band of socially acceptable behavior for women is still narrower for women than for men, studies show. Until that changes, the power of women, at least publicly, will not equal that of the guys. And the pay won’t either.