WoW does the same thing, except due to the way the engine's built the cap was at like 214thousand for a while, it all depends on how they code the core really. Unsigned Int, bytes, however they set it to be in the database is how it'll acquire it's limit.
As for becoming a negative when you went over, that was probably faulty coding on the private servers behalf, that's not supposed to happen in Lineage (have many friends that hit the limit in the real game) and it just stops collecting like Vindictus' AP.