Render passes 8 bit brightness problem

[Dmi3ryd]

Hello.

I made a picture with render passes and save as 8bit png each pass.

Then I made compose all passes got the difference in brightness.
I set blending mode to Add.

Composed and beauty render (octane) different.

Could you tell me please why composed image is brighter as original octane render?

Octane settings that I used when save image:

expose 1
Linear/Off
Gamma 2.2