Funny behaviour: Samples bonus for old GPUs

[Sylk]

For some reasons, my old 570 continues computation, while the 1080 computing is already finished and samples count reached. See vid:

[Karba]

because 570 is too slow.
it takes some samples to render and while it is rendering, 1080 does the rest while 570 still rendering. when 570 finishes taken samples, maximum samples are reached by 1080, but 570 has to store its own rendered samples at the end.