Downvotes are given for any reason the downvoter wishes (as long as not abusive, which AFAIK is defined as targeting a specific user, regardless of the quality of the question or answer).
See this meta question -- the consensus is clearly that:
Users Should Downvote However They Want
That said, after a user downvotes, if the question is modified, the user may or may not notice, and may or may not change their vote. Period. The system does not force them to.
If a post has been improved, generally there will be more upvotes at that point, whether or not the downvoters change their vote. If the total remains low, or there are more downvotes, the consensus is that the post is not high-quality,
regardless of the opinion of the poster.
This is how the system works. A posts quality is judged by consensus. Is the system perfect? No. Does it have to be perfect? Also no. Can it be perfect? Most likely not.
But if a member finds that one or more posts highly downvoted, this is an indication that the posts do not find favor in the eyes of the community. It is not personal, please do not take it that way. Either learn to write posts that the community favors, or find a different community which appreciates that style of post. No community appreciates everything.
Up and downvotes are not payment and debit. Don't take them to heart too much; they are just a guide.