How do stats from chems / aid items stack? How do they add up?

I've noticed that chem stacking isn't as straightforward as I initially imagined. I did the following experiment:

S  P E C I A L
6  5 4 5 6 7 2 -> initial stats without any chem consumption
8  7 6 5 8 7 2 -> buffout + mentats
9 10 7 5 8 7 2 -> buffout + mentats + bufftats
9 13 7 5 8 7 2 -> buffout + mentats + bufftats + orange mentats

So I guess what I'm asking is, what gives? What's up with that perception outlier?

fallout-4

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Do Chems stack fo4?

Stacking Chem EffectsYou CAN stack beneficial effects if the base drugs are different. For example, Psychobuff and Buffjet work fine together, because the base (first) drugs are different.

Do Chems stack Fallout 2?

- chems do not stack; only one may be active at a time.

Do the effects of Mentats stack?

Effects from Party Time Mentats stack with regular Mentats, but Mentats do not do so by themselves. Party Time Mentats also have a drastically decreased effect time of 1 min, a 75% decrease from standard Mentats.

FALLOUT 4: The ULTIMATE Chems Guide! (Everything You Need to Know About Chems in Fallout 4)

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