Well, for 20 upvotes at full power per day, 60% is the initial voting power that you have every day when you do your 20 upvotes in rapid succession.
When you want to do your 20 upvotes at the same voting power, you need to space them equally, and your voting power will be 50% for each upvote:
From voting power 50%, after one upvote, your voting power is 49%. After 1 hour and 12 minutes (= one day/20), your voting power is back to 50% and you may vote again.
These are the two extremes: 50% and 60%.
I discussed it with someone who is a lot smarter than me on curation and you are right. I appreciate you helping to clarify this!