This will be mainly due to the new "CLRC" reward curve.

• If you upvote on a post / comment with zero payout your vote will be 50% of what it was under HF20.
• However if that post / comment payout grows to (something like) 20 Steem then your upvote value will actually have increased back to 100% of what it was under HF20.
• If the payout of the post / comment gets very big your upvote will be worth more than it was under HF20.

This ignores other (I think smaller) complications introduced under HF21.