Odd difference in spiritbonding items

Hi,

I've been spiritbonding items for a couple of days, mostly in Castrum, all with HQ jewels.

Oddly, my jewels were all ilvl 48 (a mix of zircon/ spinel and such), all HQ, but the spiritbonding at the end was still at 89% for one, 91% for anoter, while being 100% for the others.
Hadn't bothered melding crap materias on any of them either, so any idea where this difference comes from ?

There are no level 48 earrings, and the only ilvl 48 rings aren't something you would be spiritbonding.