Yeah, it's hardcoded into the EXE as a table. For each level the table entry can either point directly to a levelXXX file and position within file, or I think it can point to an entry in ODDTABLE.DAT for repeated level (where the ODDTABLE file will continue contain the changed stats for the repeat--note that "repeat" in most case is actually the earlier, easiest appearance of the level).
It's been a while since I worked it out so I'll have to dig around to find out more details. I'm curious, why does it matter to you?