I feel the important piece to remember with VO2Max estimation is: Its an estimation. Its significant figures [1]; reporting the value to one or more decimal places communicates a level of confidence inappropriate for how inaccurate these estimations generally are. Especially the Apple Watch's; Garmin's is known for being pretty decent, usually +/- 2, but Apple Watch's is all over the place and is infamous for being really inaccurate.
Clamping pace to 5 seconds is a similar idea. GPS isn't super accurate: within 16 feet some sources say [2], though it gets better if you've got dual band, if you're moving; but it gets worse when you don't have an open sky. Just ten feet of GPS inaccuracy over a ten minute mile means your recorded pace is somewhere between 9:58/mile to 10:02/mile. And, experimentally, these systems are way, way more inaccurate than that: on a recent bike ride, with no major sky obstructions, I wore both an Apple Watch Ultra 2 and Garmin Enduro 3; the AWU2 recorded 25.05 miles, Enduro 3 recorded 25.18 miles. That's a difference of ~686 feet; ~27 feet/mile.
Clamping pace to 5 seconds is a similar idea. GPS isn't super accurate: within 16 feet some sources say [2], though it gets better if you've got dual band, if you're moving; but it gets worse when you don't have an open sky. Just ten feet of GPS inaccuracy over a ten minute mile means your recorded pace is somewhere between 9:58/mile to 10:02/mile. And, experimentally, these systems are way, way more inaccurate than that: on a recent bike ride, with no major sky obstructions, I wore both an Apple Watch Ultra 2 and Garmin Enduro 3; the AWU2 recorded 25.05 miles, Enduro 3 recorded 25.18 miles. That's a difference of ~686 feet; ~27 feet/mile.
[1] https://en.wikipedia.org/wiki/Significant_figures
[2] https://www.gps.gov/systems/gps/performance/accuracy/