Actually, WN#59 is always a 9:00 PM - 10:00 PM flight from DAL to HOU, so if this flight had a PHX-DEN leg, it must have been earlier in the day - maybe PHX-DEN-AMA-DAL-HOU? Currently, flight #59 is only DAL-HOU, but it often has one or more legs prior to arriving in Dallas.

For the past several (at least 20) years, flights 1-63ish with odd numbers have traditionally been assigned to DAL-HOU flights, with flight #1 currently departing at 7:00 AM, #3 departing at 7:30, etc. on every half hour. Even numbered flights 2-64ish all include a HOU-DAL leg, with flight #2 leaving HOU at 6:30 AM. The first flight out of HRL in the morning is often flight #8, with a schedule of 6:35 HRL-HOU 7:35; 8:00 HOU-DAL 9:00; and onward from there.

There are several other patterns one can find in the Southwest flight numbering scheme. For example, many intra-Texas flights are in the 160-240 range.