I'm guessing that this is a serious question, so I'm going to give it a serious answer. This is a classic economic signalling behaviour, akin to getting a college degree in order to signal your intelligence or giving gifts in order to signal your wealth.
Consider two possible YCs: YC(a) is serious about having female and minority founders at office hours, while YC(b) is just trying to be politically correct. Advertising "we encourage female and minority founders to apply" wouldn't cost anything. Advertising "we will only accept female / minority founders in these months" in contrast has a cost -- they lose the opportunity to meet truly awesome founders who do not belong to the appropriate group -- but for YC(a) that cost is partially balanced by the fact that they're spending time talking to members of groups which they would like to help out. Thus the net cost for YC(a) is lower than the net cost for YC(b) -- just like getting a college degree takes less effort for someone who is highly intelligent than it does for someone who is less intelligent -- and so YC communicates to founders that they are more likely to be YC(a) than YC(b).
(Whether anyone thought it through to this extent, I don't know. But people routinely engage in signalling behaviours without being aware that they are doing so.)