Could be due to a/c design. The premier intercontinental a/c of the '70s-'90s, the 747 is one of the fastest a/c (cruises at mach 0.85 or so) out there but also fairly thirsty and becoming increasingly rare. Trijets, more efficient twins (and possibly) slightly-more efficient A340s have slower cruise speeds (around/above mach 0.80) though I think the 777 can cruise at mach 0.84.