I have acquired over a long time period several NOS military and commercial barrels for SR Mausers. Five in all. Never mounted on a receiver. The literature states the greater thread diameter should be .980 inch. However my measurements with two different calipers and one micrometer is .995 for all five. I also have two M98 NOS barrels. A M1908 Brazilian DWM barrel measures 1.110 not 1.100. But a Yugo 8mm bbl measures 1.05. I will add that I have used controls such as reloading bullets and drill bits to to establish proper calibration of measuring instruments.

At the shop I work at, we do not mount barrels so I have no experience with this process other than what I glean from the literature and forums. Is this common with new replacements? During the mounting process are these barrels usually further machined to the receiver or do I have some out-of-spec bbls?

What you are seeing is normal; they vary from the "standard" of .980 and 1.1.
Many commercial barrels are loose, so one size will fit everything, and some military replacements are tight; some too tight. Lots of these install it yourself barrels have been made since the 50s by all manner of shops that definitely made sure they were small enough to fit anything, and surprisingly they do manage to work.
And they were indeed inch spec'd from the start. Not metric.
Fortunately 12 TPI is very deep and forgiving (if not perfect). If you want a tight fit you have to cut each one.