Meg -- You said it very well! That is certainly a possibility. I like Kery's point too.
Who these participants are is such an important issue. We can speculate until the cows come home about why the results turned out that way (and personally, I find that to be FUN!), but we can't know the degree to which the results would generalize to other groups unless that research gets done!
And the cool part, is that this is exactly how science progresses. People read a study and have exactly the same questions we do -- and then they go on and replicate (repeat) the study, except this time they use, for instance, the same group from the original study and then add OTHER groups, to see what differences emerge -- and also to see if those original results emerge a second time.
Can you all tell I get really excited about this stuff??!!