A lot of homeless people (not all, no where's near all, but enough to be noticeable) do have issues, be they drinking/drug problems, or mental issues, so it is very possible that they weren't fully there when they Sir'd you. That also being said, there are some who are ashamed of having to ask for help, thus avoid looking directly at someone and default to Sir.
Though looking through this thread one thing makes me cringe, and that is the noticeable opinion that homeless people are to blame for their own situation, while this may be the case for at least half of the homeless population, it is not the case for all who are homeless. There are some who are disabled and fighting to survive without much luck or family to help them while fighting to try to get assistance from the organizations who offer it (those organizations are so use to people taking advantage of the handouts that they sometimes make getting help hard for those who really need it to discourage the lazy who don't need it from taking advantage.) or just ended up down on their luck, I know, I was on the streets for two years myself due to being young with no safe place to turn for help and no experience to help me make my way. I got out that situation, but it took me two years to get on stable footing. So please don't judge all homeless people the same, some of them are there due to their own stupidity yes, but not all of them are, some really are trying. Sadly unless someone smells of booze there's no way of telling which is which, thus why I normally just advise them on the location of a nearby foodbank to try to help them along and leave it at that.
But anyways, back to the topic at hand. It is very possible they just didn't take a good look or were in an altered mental state when they Sir'd you, don't take it as a measure of how well you pass. If you normally pass alright, then there you go, even bio-females get Sir'd sometimes, look at the majority of encounters, not the minority, and don't let getting Sir'd by someone who may not have really been looking at you, properly at least, get you down.