Hi,
It is sort of a bug but also by intention. The plugin will try to calculate the possible results matched by the current filter setup. However on multisite it’s a bit different as it requires multiple blog switches and many other very resource heavy things.
So there is a “guessing” algorithm involved to help out the peformance, and it is unfortunately in these cases not very accurate. If the fully correct count was calculated, that would very likely bring down the website as it takes too much time. It sounds stupid I know, but it is what it is.