Glock is doing their corporate CYA-routine. If you're loading near max, based on +P load data, right on the edge of blowing up your Glock, loading 0.01-in deeper could possibly become a safety issue.
If you're loading to standard pressure, and also loading a max-pressure load, reducing your OAL by 0.01 could take your ammunition over the edge from standard pressure to +P.
I intentionally select slower powders that provide the velocity I want to achieve with a significant margin of safety that I'm not losing sleep over seating depth or over normal variations related to typical powder dispensing systems.
I'm not trying to save $0.00000019 per round by shooting smaller charges of faster (higher-pressure) powders.
BTW: if you were to measure every powder charge you drop on a progressive press, you will see what variations can occur. If you're loading at max with a fast powder, there is some level of probable risk that your pet load may unexpectedly result in a load that exceeds that published maximum.
Also, don't forget, the SAAMI max is based upon an AVERAGE max pressure, not an absolute Max Pressure.