Jim l have worked with quite a few Winchester barrels over the years in 7mm and 300 win mag and have noticed a tendency for them to take somewhat heavier charges to get to velocity as you noticed with your 175-7mm combo.
This is not unique to the 7 Rem Mag.
It may be due to throat length and rifling and might be a built in goof factor to be certain the rifle per ft orms and functions with the impossibly wide range of factory ammo available for popular cartridges. They never know what ammo will br used.
These factors have an effect on the velocities we see and why your rifle might accept heavier ch as rges than those in the manuals for the same veloc ities.
Lik l saud l have akso seen this with the 300 win mag.
