I wrote about the confusion surrounding calculating upvote values before upvoting - here. Also explained by @steemchiller, I see, in comments.

Essentially, every platform now needs to make an estimate and many just choose to use different estimates.

As @steemchiller also noted, the vote weights are no longer broadcast, so that any platform needs to simulate the full reward curve formula as each vote takes place. This is also no guarantee as steem uses a number of approximations to handle the huge rshares numbers.

Anyway, hope some of this helps.