I will try to explain the problem with mounting scopes.
Everything made by man is manufactured to a tolerance. It can range from that is good enough to millionths of an inch or the decimal equivalent.
Stacking tolerances are when the tolerances all lean the same direction, all positive or all negative.
The first issue is that many rifles were manufactured by various countries even though they may all be called the same thing.
The second issue is that the holes, whether factory placed or put in by a garage mechanic, most likely are not perfectly aligned.
Thirdly someone else made the bases to fit the ideal receiver contour and hole spacing, and they also produced the bases to a tolerance.
Fourth, someone else made the rings to a plus or minus tolerance.
Standard manufacturing tolerance is .030. If you are talking about a surplus military receiver, that tolerance has most likely been exceeded.
Let us assume that a tolerance of .010 was held throughout the manufacture of everything.
On a two piece base set up, if the front tolerances add up to .040+, and the rear base tolerances add up to .040-, there is a .080 misalignment multiplied by the distance between the rings.
In most instances the manufacturing tolerances negate one another. That is a .010+ base plus a .010- ring cancel each other. But this is not always the case.
I have seen instances where just switching the front and rear bases would decrease misalignment. I have also seen cases where I would have several packages of bases open trying to find one that would negate misalignment.
At the extreme, I have had to drill and contour a piece of steel to fit the receiver, align the axis of the receiver on the mill, and cut the bases to be in alignment with the axis of the receiver.