Loft and CLO are two different things... for instance, 2 inches of loft for a particular insulation will always be warmer than 1.5 inches of the SAME insulation.
You can't compare 1.5 inches of one insulation type with 2 of a totally different type unless you have a reference number (such as R-value or CLO).
In the case you've outlined, the HL would have a total CLO of 3.4 The XP has a total CLO of 4.1.
SO.... the XP is about 20% better insulation, if the numbers you give are correct, even though the HL is thicker!
One of the nice things about CLO is that it relates insulation value to WEIGHT. There are more efficient PER THICKNESS insulations out there, but they might not be as efficient PER OUNCE, and when backpacking the weight makes all the difference.
Hope this makes it clear, and that I've done my math correctly. :P
Happy New Year!!!