It seems like you're assuming that all of the coins in a single UTXO will stake. When I stake with my wallet though, that's not the case - only a portion of my coins stake.

I assume that wallet balance is a sum of UTXOs and if it stakes one of the UTXOs is locked for 16 hours (mature time).
I don't fully understand mechanics and there is no good documentation - the only way is to go through the code, but I'm not so fluent plus time constraints...