it changed the values (as can be seen from options/get), but it didn't have any practical effect. I'd argue that if this is the expected result, than the setting should be considered read only and return an error if one tries to modify it.
If the sync has already started it won't change the number of Transfers it is using - there is quite a lot of setup for each transfer (threads started etc) so it isn't trivial to change the number.
It should take effect for the next transfer though.