Remember, as far as we know BS only has a strength list with 21 levels on it. By all accounts, he doesn't really know HOW MUCH stronger one channeler is than another. As in, he knows channelers are 1 or 2 or 5 levels stronger than other channelers, but he doesn't know whether they're 25% stronger, 50% stronger or 100% stronger.

So, when BS first made the comment about Cyndane being VERY weak, it was because he still didn't have Maria's understanding of the VAST strength gap between Lanfear and Moiraine.

All BS knew, was that the amount of Power that Lanfear lost, was enough to make Moiraine stronger than say Cadsuane.

So, if BS thought Cadsuane was a pretty strong channeler, say close to Forsaken strength, then he would think that for Moiraine to gain so much, and Lanfear to lose so much, would make Cyndane very weak.

As an example, I believe that if Lanfear has 100 strength, then Cadsuane has around 35 and Moiraine has around 30. But if BS had an incorrect understanding of this, he might have thought that if Lanfear is at 100, then Cadsuane is at 80 and Moiraine is at 50. So, for Moiraine to exceed Cadsuane, she might have to go up to 90, a gain of 40 units of strength. If that same amount had to come off of Lanfear, then Cyndane would now be on only 60 strength.

In reality, however, we know that Moiraine doesn't have to gain much to surpass Cadsuane, and therefore Cyndane doesn't have to be much lower than Lanfear to have transferred that portion of her strength to Moiraine.