The daily vote limit is a little arcane. From this post on Meta.SE:
- The exact number of votes available depends on your voting behavior for the day. Concretely, there are three upper limits to respect at the same time to be able to vote.
- 40 post votes per day per user.
- 30 answer votes per day per user.
- 5 post votes after you got your first "5 votes remaining" warning of the day. If you reach 25 answer votes at time
t of the day, given
q(t) as the number of question votes cast by time
t, the total post votes limit will be no more than
30 + q(t). If you cast less than 25 answer votes, ignore this point.
So, basically, you get 10 votes specifically for questions, but you need to use them early. Once you get close to the limit of 30 unrestricted votes, you can't claim all 10 of the others any more.
No, I don't know why. I suspect it's a limitation in how voting was implemented (the 10 extra votes for questions were a later addition), rather than an intentional design decision.