For ease of reference, I adapt a part of my answer to meta.SO post Why is deleting own posts consuming delete votes..?
Each 10K users has two kinds of delete votes:
- all-purpose delete votes (between 5 and 30, the number depends on reputation)
- self-delete votes (5; these are available under 10K too)
The software uses all-purpose delete votes as long as you have them, even if you are deleting your own post. This means that to avoid wasting the delete votes on self-deletes, one should postpone them until there are no all-purpose votes left for the day.
To calculate the number of remaining delete votes, software subtracts the total number of delete votes cast so far from the number of all-purpose delete votes available. The result may be negative, as you observed.