Think of it like
A coffee shop. One person takes orders, one makes the drinks, one runs them out to tables — all at once. So when the order-taker gets stuck on a fussy order, the drinks already in progress still reach their tables; the line just can't place anything new until they're free again.
You'll hit this when
Your screen keeps scrolling smoothly, but the second some heavy JavaScript runs, taps stop landing and new rows flash up blank — until it finishes.