You were shooting a 300 gr bullet with a high BC.
The math is likely correct(I won’t say always as we are continually refining formulas), but hitting a high pressure area or a wind pocket with the smaller cartridge is going to have a greater effect.
You also can not compare say a 6mm, 308, and a 338 lm to one another at the same distance (say 1k yds).
These cartridges had very different trajectory.
The fair comparison would be on same day with same conditions and at distances which the trajectory (lets just say max ord) is the same. This would let all three bullets enter the same wind zone as each other.
So, you might had to run 800, 1k, and 1700 for .308, 6mm, and 338lm respectively to get a fair comparison.
And even then, what the wind is doing in a zone 20 feet in the air at 600 is going to be different at 1500 20 feet in the air.
But without a metic shit ton of data and some extremely accurate device (lidar for example), you cannot even begin to challenge the math.
I did edit the post to bring the 7 SAUM into the picture as I knew you would bring up the point of bc and comparing cartridges. I shot .338 for years and never had to make corrections that software suggested. I have however for everything else and I'm aware that all the situations are different.
Again, I don't want to argue about the math and call it wrong although it is constantly evolving, just that sometimes things happen that the math can't account for.