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?

I was only going to post the question but I ended up lifting some weights with my liver for science, and I think I came up with the answer (if someone knows anything else please do tell).

The answer is:

1.- There are chem (aka aid items in general) classes. For instance, wine and whiskey are both alcohols, and normal mentats and orange mentats are both mentats. I do not know all chem classes.

2.- Chems from different classes stack, fully.

3.- Chems from the same class do the following: if each chem changes different stats, both changes are kept. If the same stat is changed, the effective value is that of the last chem taken, meaning order matters. I don't know yet if this also affects the length of the effect.

To illustrate, I'll show the results of the experiments I made, chems shown in the order I took them:

Experiment 1:

Reminder of the effects of chems used:
Buffout  -> +2 STR, +2 END, +50HP
Mentats  -> +2 INT, +2 PER
Bufftats -> +3 STR, +3 END, +3 PER, +65HP
Orange Mentats -> +5 PER, +10 accuracy in VATS

S  P E C I A L
6  5 4 5 6 7 2 -> initial stats of my character
9  8 7 5 6 7 2 -> bufftats
8  8 6 5 6 7 2 -> bufftats + buffout    (buffout overwrites strength and endurance. Perception from bufftats is kept.)
8 10 6 5 8 7 2 -> bufftats + buffout + mentats    (bufftats and mentats stack)
8 13 6 5 8 7 2 -> bufftats + buffout + mentats + orange mentats    (orange tats overwrite mentats but not bufftats, so 5+3. Int from the mentats is kept.)
8 10 6 5 8 7 2 -> bufftats + buffout + mentats + orange mentats + mentats    (mentats overwrite orange tats)

Experiment 2:

Beer -> +1 CHR +1 STR -1 INT
Whiskey -> +2 STR -1 INT

Taking nothing: +0 CHR, +0 STR, +0 INT
Drinking whiskey: +0 CHR, +2 STR, -1 INT
Then drinking beer: +1 CHR, +1 STR, -1 INT
Then whiskey again: +1 CHR, +2 STR, -1 INT

So remember to take your meds in order. I'm off to invest in addictol now.