I'd bet it is a tight bore. I've also read of guys who had to bump up their favorite handload a bit when moving from a custom barrel to a factory barrel, claiming the factory bore was "looser" and needed a higher charge to make the same speed.
On the other end of the spectrum, I have a Lilja-barreled 24" 7mmRM that was really slow with book loads. The throat on that rifle is super long, almost like a 7mmWby. When I worked up to typical upper-end 7mmRM velocities, I was using charges approaching 7mmWby data. One load manual indicated I was something like 7gr over max, although 7mmRM data at the time was all over the place when looking at charges. I've been shooting those loads for years now with no issues (160s at 3,000fps with RL22).
