Its two main reasons:
- Like you said, a fixed reward pool.
- People who only vote 5x a day (freds) are getting a huge bonus.
Right now, infrequent voters are leaving a huge amount of meat on the bone. The change to a 5 vote target effectively optimizes them. The extra money infulence they get has to come from somewhere.
Now the question is, how many of them are there? Its tough to speculate. More than none, for sure.