So, I was curious about this, too- and when you great-circle map them (which probably makes sense from an aviation aspect, as while it's never "as the crow flies" due to traffic patterns and such, it's a better representation of distance than roads are).

Turns out that while say, LAX-ONT is 47 GC miles, SNA-SAN is 76 GC miles (despite the fact that I'll bet on most days you could drive SAN-SNA faster than you could LAX-ONT).

TPA-MCO is 81 GC miles and AUS-SAT is is 66 GC miles. It wouldn't surprise me if WN used a ~50mi limit for co-terminal determination.