Immediately before casting my 40th vote at the end of yesterday (GMT) the close vote dialog showed 11 votes remaining:
After casting that vote I was shown that I'd hit my 40/day maximum:
And now after the daily reset it's showing that I have 50 votes again:
Is this a bug, or is something else going on?
50 is the number of actual close votes you can cast: this includes not only those cast from making "Close" reviews in the review queue, but also those you cast externally outside the review UI.
40 is the number of close reviews you can make per day: this includes all review outcomes, including both "Close" and "Leave Open", but does not include close votes you cast outside the review UI.
On most sites, the two above numbers are 24 and (usually) 20, respectively, but on Stack Overflow, the daily close vote allowance is set to 50, and the size of the queue is above a threshold, giving users the ability to make 40 reviews instead of the usual 20.
The two numbers are different because they represent two different things.
You're getting close votes confused with close reviews.
You have a maximum of 50 close votes per day on Stack Overflow. This seems to be functioning properly since you had 10 left after casting your 40th close vote.
Since the Close Votes queue is generally overflowing on Stack Overflow, you get a maximum of 40 close reviews per day.
You used up all your daily close vote reviews, but not close votes.
