It is due to daylight savings time. Qantas time it so that it always is scheduled to arrive in DFW at 13:45 (in part) so that onward connections are not changing.
The scheduled time is not the same as flight time. For example, if it is leaving SYD during a busy time of day, they may allow extra time to get airborne. Also, the prevailing winds change based on the season, so there actually can be a variation in average flight time through the year.