This is due to the difference in the FOV of your source. A monitor distance based on say 55 vdeg will be different than one based on 120 hdeg. For this reason you should stick to one source for all your conversions. Ideally you should match the hipfire FOV of all your games, but that's not possible as a lot of games have locked FOV.
You would want a scope with a FOV of for instance 20 hdeg to feel the same for any game, regardless of what the hipfire FOV is, that's why you should use the same source.
Note that if you use monitor distance for all conversions (hipfire, ads, scope) then it doesn't matter what your source is as long as it is derived from the same conversion (although rounding and lack of decimals may play a part). The issue only comes when you use 360 distance for hipfire and monitor distance for ads and scope.