Dva Combo of "Pew Pew Pew" and Headshot maybe calculating wrong damage?

I am not certain if this is a bug or if I am not interpreting these talents correctly. PPP fires 3 times, and “Each shot deals 75% damage”.
Headshot: Increase Big Shot’s damage by 75%…

At level 20, Big Shot states its normal damage as 329. With PPP alone, one would expect each shot to be 329 x 0.75 = 246.75, and testing it shows it as 247, so that looks good. With HS alone, one would expect the damage to be 329 x 1.75 = 575.75, and testing shows it as 576 so that looks correct.

But when I use both, I would expect each shot to be 329 x 0.75 x 1.75 = 431.8125. But testing the combo shows damage of 493 on each shot. I can’t for the life of me figure out how it comes up with 493 instead of 432. I made sure the target didn’t have any armor change to cause it, and specifically avoided talents that affected this damage.

I noticed that 493 is almost exactly 1.5 times that of 329. Could if be that the 75 increase of HS is being added to the 75 of PPP, making each shot 150% of base value?

I also see that 493 is almost exactly twice that of 247, which is the 75% damage value. Could it be that the 75% value is being then increased by 100% rather than 75% due to HS?

I just don’t know where it comes up with 493 for each shot in combo of these talents.

Both of these are additive bonuses, which mean they modify the value based off of the base value.

Like 80% of damage modifiers in the game are additive, and additive damage modifiers also cap out at -75%, but have no upward cap.

For another example of this, as Zagara, pick both Medusa Blades(Level 2) and Jagged Barbs(Level 16), and you’ll see that the 40% bonus damage from Jagged Barbs modifies the damage dealt by Medusa Blades as though it deals the same as the base damage, causing those Jagged-affected Medusa Blades to deal 27% less damage than your standard basic attacks(note that your basic attacks while doing this will also be increased by Jagged Barbs), so compare unjagged-basics to jagged medusa.

An example of a multiplicative modifier can be seen on Jaina’s Wintermute, where the damage dealt by the Water Elemental is 50% less than standard, and Jaina’s Chilled damage modifier is additive.

Prior to the rework on PTR, Pew Pew Pew’s damage modifier was multiplicative, but with the rework, it was updated to be additive. So the math here is 329 x (0.75, 1.75), and if you had some other additive modifier like Shrink Ray, that would be put in alongside the same bit as PPP and Headshot, making it 329 x (0.75, 1.75, 0.5), which would equal out as doing the same damage as base, as everything cancels each other out.

I guess that makes sense as the first thing I suspected of it effectively being additive. But that does not seem intuitive to me. Maybe if Headshot read “Increase Big Shot’s damage by 75% of base value” it would have been more obvious.

I don’t think it needed to be explained that it is additive, because you would have to write it on so many talents it would become almost annoying to read about it.
Almost all % damage are additive, but % bonuses multiply flat bonuses:
fenix lvl 1 plasma cutter build gives +60, lvl 7 gives +40% damage and they interact with each other, but if you pick +15% damage at lvl 16 it adds for a +55% together with the lvl 7 one (still interacting with lvl 1 quest).
Fenix is a good example because you also see that attack speed modifiers are additive: lvl 7 warp warfare says: gain +175% attack speed in pewpew mode, but you don’t effectively even double the attack speed, because pewpew mode has a baseline +150%: so you go from +150% to +325%, resulting in 425/250 ratio which is a effective +70% real bonus attack speed.
Also all spell power modifiers are additive, i.e. Guldan’s lvl 7 + lvl 16 = + 40% spell power.

Hopefully i provided enough examples to switch the calculations on other heroes.