I am not sure on the non-refund of the additional amount if revote value is less than the original one.
Voting power should never be refunded. Once a vote is cast, the voting power is used. End of story.
What I would like to see changed though, is if you vote 10% and want to change it to 20%, it doesn't cost 30%. As long as the vote time is adjusted to the latest vote, there is no way to exploit it for personal gain. I would like to see a future votes only cost the difference in %, provided the vote is going up in %. Going down, nothing changes as there is no scenario where voting power should be refunded.
That's fine about no refund. I wonder anyone would do that.