I thought it was the effects of the scope expanding not just the gun or mounts, that's why some target scopes are silver to reflect the heat in higher temperatures.
All metals have their own expansion rate (co efficient of linear expansion) so expand at different rates. I did listen to some things at school!
I was going to say I had not noticed this in lower mag scopes for HFT but then remembered a lot of us with Steyrs suffered POI drop at distance with 10 mag when the weather turned cold last year, but this could just have been down to air density.
In answer to Andys original question, yes I think you are right. Different matarials in the rails and mounts etc could make a difference. I suppose you could measure them all if you could be accurate enough.