Do you have any idea why the SBD amount on a post can drop ? With no flags on that post ?

It can't be SBD price fluctuation.

I found this, but it makes no sense:

https://steemit.com/steemit-howto/@dr2073/why-sometimes-steem-reward-go-down

The answer to which the original poster is satisfied is this:

"The amount of reward you get per vote is based on the vesting Steem of the person voting, and the more they vote on different things within the same 24 hour period, the more the value of each vote gets diluted. So after voting on your post, if someone moves along and votes for a few other posts, the value of the vote they originally put on your post drops."

Which makes absolutely no sense, since the SBD amount is already given to a post it can't decrease, even if the upvoter totally depletes his/her account.

As another user put it in the same post:

"it’s the same as if I bought bread in one store with some magic money and then milk in another store, and the money in the baker’s cash register would disappear just because I bought milk somewhere else."