It's commonly because all votes are not created equal. The system is designed to favor votes on questions, and the simplest way I have seen it explained is that
- Everyone gets a total of 30 votes.
- You may get up to 10 extra question votes.
- Warnings remain simple... (5 votes remaining, 4 votes remaining...)
- Question votes may be cast at any time during the day. However, once you reach the N votes remaining warnings, stuff is set in stone. A question voting spree at the end will still count down. [sez waffles]
Put another way, Shog9's formula offers that the number of total votes you can cast on a day is
min( 30 + ⌊1.2×Q25⌋, 40 )
where Q25 is the number of question votes amongst your first 25 votes and ⌊⌋ denotes rounding down.
For complete details, see What are the limits on how I can cast, change, and retract votes? at MSE.
Votes on deleted posts do not count toward your daily total, thus How does someone get to cast more than 40 votes in a day?