CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
A longer bullet ogive can do it, too, by allowing more starting gas to bypass the bullet and head down the barrel before the bullet obturates the bore. That reduces pressure. I note from photos of Hornady's test setup that they have what looks like pressure-measuring instrumentation on the barrels. However, I believe they still load to velocity intervals and then use the last interval load before one when over SAAMI pressure. Some of those will be close, and some will be nearly a full interval lower, so the peak pressures aren't necessarily very close to SAAMI max. IIRC, when I looked at that some years back, it appeared their top loads in a few cartridges I looked at (not exhaustive) varied from about 1% to 9% below SAAMI max. I would expect Sierra and anyone else posting charge weights in velocity intervals would have the same thing occur in their data.
I vaguely recall hearing there was some point in the Weatherby's history when the long freebore was changed, but I don't recall which way or when. Perhaps someone with more knowledge than I have about Weatherbys can chime in and sort that out. I mention it simply because it could be a factor in pressure discrepancies.
CIP shows the 375 Weatherby freebore as about 0.36 inches. Using that much bullet jump in GRT (which has this feature while QL does not) drops the pressure of the 90-grain load from roughly 74,000 psi to 66,000 psi. Add to that a slightly large chamber, and the OP's loads could score the 63,800 psi (4400 bar) limit the CIP has for its average peak pressure. So, it's possible this is still good to go, though pushing powder charge and pressures will always erode throats and hasten the end of barrel life. There is no free lunch. You also still have to consider the OP's individual lot number of IMR4350 could be a little slower than average, too. Lots of variables.
It's too bad Jim Ristow had to shut down his Pressure Trace business (health reasons, not lack of product interest), as that was the best way for a handloader to check the pressure in an individual gun.
40 to 50 shots a day? That's a lot of big bang to get hammered with repeatedly. I can do about a dozen with the 416 wby mag but don't go beyond that since my ability to maintain good accuracy will fall away after that--just the concussive report alone starts to jangle my skull and what little is in it. LOL