ALL deconstruction XP is capped by the level of your craft. If you paid attention, you would've noticed this in BS, WW, Clothing, and Enchanting.
It simply means that your level in the craft is so low that every kind of jewelry that you deconned exceeded that cap, so they all appear to give the same XP.
Up until somewhere in the 20's, any CP160 jewelry you decon will exceed the cap. I usually save purples for at least level 30 in the craft, and intricates for level 40 in the craft.
Which part of my post (that consists of ONE SENTENCE) where I say "regardless of crafting level" did you not understand?
Seeing as how I was just leveling jewelry crafting on a character today and didn't see what you reported--and have never seen what you reported on the 8 other characters that I've taken to 50--I can either take "regardless of crafting level" to mean the character's level or the jewelry's level. The latter seemed much more likely, particularly given the lack of specific detail.
This being my fifth character I am levelling jewellry on, I'd say I've seen enough as well.
It was especially obvious when I was at level 45+ and was getting the same xp from blue and purple items.