Pickups, pickup placement, picking position, pick material, hands, amp and pedal set up...if any.
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These two P90 SGs sound very different from each other. They sounded different when they both had the same bridges too. When both had stock Gibson P90s they sounded different.
Each of the two has its own personality(sound wise). The small guard has the pickups spread wider apart...the bridge pickup is closer to the bridge, and the neck pickup is closer(very slightly) to the neck. This guitar is a bit more brash sounding. It has more contrast in tone, between the two pickups, than the black batwing.
The batwing sounds more civilized, smooth, when changing pickups. The bridge pickup is easy to manage with higher gain.
The batwing, I am comfortable using to play anything in my wheelhouse. The small guard is more like my “punk rock”, edgy, SG...or my wooly, 70s fuzz/muff machine. If I’m attentive with the tone controls, I can keep them sounding fairly close to each other tonally....they’re just two different beasts really...and I’m always surprised as to how different, when I play them back to back. It ain’t the paint that makes the difference.
Most everything I play, I sound like me. Even if I use other people's gear. That's why I put hands first. I know if I plugged into David Gilmour's or Michael Schenker's rigs, I would not sound like them.
This is what I am sayin............... I believe some fellas here will say the same about 2 ES 335 guitar they have as well.
Both basically the same guitar, yet both sound totally different than the other.
I was specifically referring to the different pickup positions in those two P90 equipped SGs, mostly...but I agree that your scenario has also been true, in my experience.

Amen Brother.I will say tone is everywhere.
Amen Brother.
I believe Jesus is everywhere; is tone Jesus?
So...if it's the amp in your configuration what is it with a unit that uses powered speakers such as the AxeFX, Kemper or Helix?
I'm running my Helix through a Carvin power amp into a 2x12 cab. So what is my biggest contributing factor?
The reason I left the 'fingers' or 'player' out of my reasoning is because its the one variable that can't be controlled. I'm keeping it down to the individual, meaning what are the things that make the biggest difference in my sound when I change them? ...when I think about what makes the biggest impact to my sound when I alter it then it's by far the amp and configuration of that amp (combo, head+cab, 1x12 vs 4x12, etc).