The in-game display of dispersion reduction contradicts tanks.gg

First time posting here, just wanna see if anybody realize this as well.

So one of the trick I‘ve learnt about tanks with bad gun handling, such as T57 heavy and obj430U, is to use bounty irm (-15%) + bond vstab (-27.5%). According to WG’s in-game display, this results in a 42.5% reduction (47.5% with vstab directive) of dispersion during movements, which improve the gun handling significantly. The magic here is that according to in-game display, irm and vstab stack in simple subtraction.

However, recently on tanks gg I found out that according to them, irm and vstab stack in multiplication. For example, batchat 25t start with an after firing dispersion factor of 4, which with bounty irm and bond vstab should be reduced to 4(1-(0.15+0.275) = 4 0.575 = 2.3; However, tanksgg says the result is 2.46, a rather underwhelming effect that fits into the model 4×0.85×0.725, which is different from what the in-game display would suggest.

TLDR: WG’s in-game display may be exaggerating the stacking effect of irm and vstab.