A couple of things to point out:
By D&D's surveys, most campaigns last 6 mos. to a year, ending between 9th-12th level.
Most of D&D's monsters are CR 9 or lower.
When you start getting into the higher levels, it's less likely you will be facing same-CR monsters and more likely you will be facing multiples of lower level opponents, save for the occasional sub-boss or boss. If you go by the encounter building guidelines, with its multiplier for # of creatures, when you actually add the XP up, it will be lower than par-CR opponents, thus slowing your advancement.
So the 1.6 you have for par-CR opponents might be more like 2.0 to 2.5 when you look at the likelyhood that the party is facing "lesser" but more numerous opponents at higher levels.