As far as I can tell,
the donations work like this:
Every day, the votes are counted from the day's blocks. The donation amount is calculated as 1% of the day's emission times the percentage of "yes, donate" votes. Then 1% of the day's emission is subtracted from the not-yet-donated amount (which starts out as 1% of the total supply). Then the donation amount is rounded down to get rid of dust.
Basically this has the same effect as if when each miner votes "yes, donate" 1% of the block reward goes to the developer, and if they vote "no" nothing goes to the developer.
You can correct me if I'm wrong.
It looks to me from looking only at that one block of code (so this may be wrong), that donations are computed from a fixed amount of the total supply (1% I guess) less the amount of previous donations. So the effect is that as long as one miner votes for the donation, the entire donation amount (1% of the total supply) will eventually be donated (over a similar timescale to the overall emission curve). Votes no don't reduce the total donation, they reduce the current donation but increase future donations, in effect slowing down donations but not ultimately changing the total.
But I haven't studied this that carefully so I welcome corrections.