Thumma preserves two items and indicates sequence with delay. Bal, in contrast, replaces or corrects the previously stated item rather than simply scheduling a later one.
In a report such as “Zayd arrived, then Amr arrived,” both arrivals remain asserted. “Zayd arrived—rather, Amr arrived” may be a correction of the first report. But a completed legal effect cannot always be treated like an amendable report.
Bal is not itself a marker of chronological delay. Its core concern is the relation of the second statement to the first.