I think that's pretty common. My Jamis has 853 main tubes only. Not sure what the rest is. Probably 725 would be my guess since they are heat treated, but not air hardened.

Ultimately, a frame builder can do whatever they want. They can say its all Reynolds and use a mix match of any supplier. They can say all True Temper S3 and slip an OX Platinum chain stay in there. Who would know?

There are other tells that would likely stop this sort of issue. If two different builders are in the business of building OX Platinum frames and one builder's frame routinely comes in at 3.7 lbs and the other routinely comes in at 5.2 lbs because he's slipping in some straight up 4130 in there, which builder will get more business?

I like Rodriquez's stance on this:
Honest bicycle weights

It reminds me of this $6000 2016 carbon fiber bike I saw on the scale recently: