I did some digging and my statement only seems to apply to long-haul flights, as mentioned in this thread [1]. "Well,most long-haul fleets fly a lot more than short-haul ones.BA 772s and VS 343s fly about 17 hours per day." That is 71%!
Average utilization is between 42% and 48%. See [2], Figure 6.
My guess is that most long-haul aircraft are flying overnight (since you have to do to a trans-oceanic route) while most short-haul aircraft are parked overnight (there aren't that many short-haul red-eye flights).
[1] http://www.airliners.net/aviation-forums/general_aviation/re... [2] http://web.mit.edu/airlines/analysis/analysis_airline_indust...