The best explanation so far is that there's some kind of bug on the site that requests less BTC than the amount I want to deposit for some weird reason.
How did you know that is a bug?
Have you tried to contact Tigergaming if what platform they use for BTC/USD calculation on withdrawal/depositing?
So that you can use that platform for calculation in the future if you are going to deposit again or withdraw.
Like the above said all platforms like Coingecko, Coinmarketcap, or preev they have different price calculations for Bitcoin.