Typically this is caused by a very high level account taking the display name of a previously lower-level account. Because Jagex does not make name change information publicly available, from the outside, this appears as if an account goes from being very low-level to very high-level in a single day.

This happens all the time without any issues, because RuneTrack has checks in place to ensure that this doesn't affect the Global Records. However, there are some very rare occurrences when a Jagex Mod manually changes a player's display name, bypassing the standard 28 day holding period of display name changes. If the conditions are right when this occurs, sometimes the name can slip through RuneTrack's detection system (because there's really no way to know for sure - as I said before, Jagex provides limited stat information to fansites) and appear to have gained ridiculous amounts of xp in a single day.

This really only seems to happen a few times a year, and the Global Records are corrected pretty quickly, thanks to posters like you who inform me of the issue.