Each round carves out 10+% for employee options, on top of 10-30+% to investors (Seed can be anywhere from 10-30%, Series A is typically 20% to just the lead, Series B 10%+).

Equity ownership and voting control are also different things. After the B you commonly have 2 investors and an independent director on the board, alongside 1-2 founders.

It can get much worse. Companies can have multiple “seed” rounds. It may not be doing well enough for a real “A” round. The naming of the rounds doesn’t matter. Valuation at each round does. If your valuation goes lower from one round to the next (“down round”) you’ll give up more equity, diluting everyone else faster.