Every record here is shaped as much by when we were looking as by what was flying. This site reads a world snapshot on a schedule rather than watching continuously, and the schedule follows visitor traffic, so some hours of the day are sampled many times more often than others. An aircraft can depart, cross an ocean and land entirely between two of our readings.
That has a specific consequence worth stating plainly: absence is not evidence. A flight missing from these tables was not necessarily absent from the sky, and a type we have identified only twice is not necessarily rare. Both may simply have flown while we were not looking. The figures are a floor on what happened, never a ceiling.
Coverage also thins geographically. ADS-B depends on volunteer receivers on the ground, so it is dense over Europe and North America and sparse over oceans, deserts and parts of Asia. A record for "the highest aircraft" is really a record for the highest aircraft over a populated area with a receiver in range.