The three traps represent three different approaches to trap placement.
"Depth-Based" fits a trap (very approximately) to the level of the party that should encounter it. "Location-Independent" uses a linear model where low-level spells are more common. Both of these use weighted probabilities for the spell class. "Gonzo," on the other hand, assigns an equal probability to each spell, regardless of its class or level.
(Character Level XXX)
Click here for the Server-Side Random Door Spell Trap Generator.
Click here for a minimal-layout PDF of the Door Traps Table.