png and jpgs with different RGB make-up

What Kevin is getting at, is that numbers are dependent on the color space used. The same visual color will yield different numbers in sRGB, Adobe RGB, ProPhoto etc."RGB" is not a color space, just a generic color mode.

If there is an embedded color profile in the file, that defines the colors. Numbers withut a specified color space are undefined.

All that said, one single digit up or down sounds much more like the result of jpeg compression. The jpeg algorithm compresses the color component much more aggressively than the luminance component, and this variation in numbers is a side effect of the compression. If you need absolutely accurate numbers, don't use jpeg.

Jpeg compression is destructive, non-reversible and cumulative.

EDIT cross post