Most healthy pilots will have no problem flying at FL120 or FL130, and some of them will have slightly impaired cognitive abilities.
And for the very same pilot, the situation could vary depending on how long ago he had his previous meal and where is the current time in his circadian cycle.
Therefore, the regulation allows for some margin.
The NCO regulation that applies to my flying is a bit more flexible; it requires the pilot in command to assess whether the level of oxygen at the flown altitude can impair the abilities of the pilot and to take measures against it.
I routinely use a finger pulse oximeter when I fly above 10,000 ft.