It could be that they have figured it out, but that they are also trying (poorly) to give you approval. I remember the first time I say a trans-woman. I figured it out right away and what I meant to do was give her a supportive little smile, but I was so surprised (understand I'd gone 18 years living in the same city, walking in the same area and never seen another trans individual) it probably came off looking a bit weird. Now when I see someone, and I can tell I just give them a little smile. So really it could be that people are figuring you out, and what they meant to do is smile at you but the surprise sort of ruined the outcome. Don't take it to heart. If anything you can feel glad that they realized, but then went on with their lives, instead of being rude.
Also, what the others have said could also be true. Ladies just get smiled at more. It was a bit weird to me being an ftm because people stopped smiling at me and conversations with men suddenly became much shorter, which was very different than what I was used to!
I don't think you should view the look as them criticizing you. Honestly we see so many people, so fleetingly every single day that I would be surprised if someone even paid that much mind (unless you really aren't passing yet, in which case just give it time). My gender when i'm wearing casual clothing is so bloody ambiguous right now but people don't look at me and go "What on earth is your gender!?" instead their brains will immediately snap to a decision, usually based on the situation in which they've seen me. At the archery range I get clocked as male 100%. at EB Games, male. I'll usually get 'ma'am' if i'm hanging out with some of my female friends.
Sorry that turned into a rant. Essentially my point is i've learned that people just don't care enough really to judge your presented gender. If they know and are aware of the trans community (more so than just knowing it's the T in LGBT) they may question your gender, but in most circumstances their brains have already determined which gender you are and have moved on. So I wouldn't worry too much, chances are everyone's just smiling at you =)