One of the most (pathetically) controversial points of the AV argument is this claim that 50% of the vote must be attained by the winning candidate. It is entirely a pedantic point, but let's look at it.
First, how does a "vote" count currently under FPTP? For your vote to be counted it must be cast, and valid. If you spoil your ballot, your vote is not counted.
Fast forward to AV. If everyone casts enough preferences that no-one "runs out" of preferences (that is that their vote is no longer transferred to another candidate) it's very clear that the winner will have over 50% of the vote. Even where preferences run out it is likely that votes will go over that 50% mark of all votes cast, preferences counted or not.
However there are situations whereby someone may well win on 50%, but not of all votes cast. Is it valid to still say they are elected on 50% of the vote?
As I said at the top, this is pathetically pedantic when coming from either side, but yes, it is still valid to claim that.
Just as under FPTP whereby if you do not vote for one of the candidates but you submit a ballot you are spoiling your ballot, if you end up running out of preferences you too are spoiling your ballot under AV. This is, of course, an active choice on your part. You are saying "I don't endorse any of these candidates, I'm not taking part in endorsing the eventual result".
As such, with these votes becoming spoiled...again, just like under FPTP...they don't get "counted" in the same sense. Recorded, of course, they were part of the process...but for the result they do not count, therefore the winner can still attain 50% of the vote..the valid vote..same as under FPTP.
In the interest of honesty, this should also mean that it's clear that there may be scenarios in AV whereby the total number of votes that someone wins on under AV does not change compared to if the vote was held under FPTP...the difference is that we know for sure under AV that the person truly held the greatest plurality, rather than just assuming it without taking in to account everyone's opinion.
So there you go, a particularly boring part of the whole discussion, highlighting how pedantic and silly it is to get in to arguments over it. "50% of the vote" is a valid phrase, at least it is if you don't go around questioning whether or not your MP currently *really* got their 30-40% of the vote under FPTP.