Since "Use CPU" is not checked and you're still getting 321 sieve tasks, I would see if venue is not an issue here. Maybe the particular computer gettig tasks is not in the same venue (location) as the one you're setting not to use CPU.
Just a guess.
"Accidit in puncto, quod non contingit in anno."
Something that does not occur in a year may, perchance, happen in a moment.