Excellent question – to which I don’t have an answer. 🙂

I suspect it’s possible that as RR geared up the Crewe and Glasgow factories they revised their own tolerances accordingly. The engines from those factories were built by largely semi-skilled labour without the years of experience that the Derby workforce had.
I’ve no idea how many Kestrels they built over the years but that was their only previous experience of anything approaching mass production. I guess even then the numbers were only in the hundreds each year.

I once had a conversation with a chap who had worked on Merlins in their service life and he reckoned that the best finished engines were the Paisley(not Glasgow) factory ones. He claimed that you could tell them apart just by looking at the finish of the exhaust ports.