I didnt know about the @ command, thx. Also didnt know there was anyway for enemies to spawn after map creation on anything lower than nightmare?
Its not anything to do with special levels anyway. I do runs without a single special level and it still happens, but I will give it another go now that I know of the @ command.
Looking at @ screen right after entering new level is a good practice as well. If you had a 100% killrate before entering a level, you know exactly how many enemies are present on the new one.
And not only that - in Hell and especially on HMP/UV, the count of enemies gives a hint about what they are (a very low number suggests a Archviles/Revenants/Mancubi packs, while a very large one suggests baron+formers packs or pain elementals with an envoy of lost souls).
And it also shows your movement, fire and reload speeds, which are always good to know.
About those special levels, try them. They are usually harder in some way than a normal level on a corresponding depth, but usually the rewards are quite nice.