Many non-philosophers read Leiter, yes; but it's worth remembering that his blog is possibly the most popular academic philosophy blog out there, I have seen it said.
(Plus, non-philosophers responding can be expected to dilute the vegetarian philosophers since vegetarian is so rare unless you have some good reason to expect the non-philosophers to be even more skewed vegetarian.)
The people who vote on a poll don't have to be regular readers. People who want to promote veganism as being ethical have an incentive to tell their friends to vote on the poll. It takes a handful of well connected vegans to get 100's of votes via twitter and facebook.
Even if you grant that the people who respond to the poll are infact philosophers, I would estimate the response rate of vegetarians and vegan to be higher than the response rate of people who don't make deliberate choices about their diet.
Information that surprises you is interesting as it exposes where you have been miscalibrated, and allows you to correct for that.
I suspect the users of LessWrong have fairly similar beliefs, so it is probable that information that has surprised you would surprise others here, so it would be useful for them if you shared them.
Example: In a discussion with a friend recently I realised I had massively miscalibrated on the percentage of the UK population who shared my beliefs on certain subjects, in general the population was far more conservative than I had expected.
In retrospect I was assuming my own personal experience was more representative than it was, even when attempting to correct for that.