I have to agree with the others that it's a perception about gender and was going to pose a similar question to the group after an experience I had this weekend.
I was doing a fairly tough loop this weekend (79 miles, 7000+ climbing). I did it solo because most of the group rides that take the hillier routes in my area are faster paced than I can keep up with. Anyway, just before the climbing starts, I had stopped at a little store to top off my water bottles and have a snack. As I'm about to leave, a group of 11 guys pull in on their bikes and it turns out their going over the same loop. I leave and about 15 minutes later, the first of them passes me, and asked how I was doing, it didn't bug me this time. I pass them later while their waiting for their slower riders (the ones who didn't pass me). They pass me again, about 30 miles later and make the comment "Good for you doing this ride". This is the one that bugged me. I was wondering if they thought it was deserving of a comment because I'm a woman or because I was solo?
"Bicycling is a big part of the future. It has to be. There's something wrong with a society that drives a car to workout in a gym." -- Bill Nye