On One Pompino owners...

actually now that I think about it there may be a chance that it was faced, I dropped it off at my LBS for them to install the headset when I first picked up the frame and I have a very hazy memory of them saying they faced the bb for free.

If they did, they would have taken a cut of no more than about 0.005" as a normal clean up cut, even that is pretty heavy. We're talking 0.040" to make any meaningful difference to the clearance.

I'm sure the difference is just down to the fairly loose tolerance on positioning the chainstay on the BB shell at the Pompino works.