Maxperson
Morkus from Orkus
The answer to that is easy. It's because sometimes you do want it to show up. I don't have the time or inclination to make a separate table for every class, subclass and level combination for 4 players x every terrain type. It's far better to make one more generic table and ignore the rolled encounters that don't apply to the particular party make-up that I'm running for.I took @TwoSix's point to be this: if you don't want Ancient Red Dragons to show up as wilderness encounters, then what are they doing on the table that you use to resolve wilderness travel? That is where the GM shows they lack the courage (ie to follow through) of their convictions (as expressed by their encounter tables).
Another possibility is that I have a generic table for mountain encounters, but I know that the particular country the party is in no longer has any dragons in it for reasons. I'm going to ignore any rolled dragon encounters because none exist to encounter.
I'm sure if I thought about it further I could come up with even more reasons why such a thing would be done.