1) The item had hit stats on it and as you upgraded to thunderforged, more of the hit got wasted resulting in a lower score. This in combination with stat weights and reforges can cause weird things to happen if you missed it.
Say for example you are just under the cap with no reforges on a normal item. You upgrade to thunderforged and suddenly you are slightly over cap and have to reforge to a lesser stat. putting you back under the hit cap. Hits value stays at a higher level than the new stat you reforged to because you are under the cap. The item is now "absolutely" worth less than it was before the reforge. (Because its looking at the higher level of hit on the normal piece compared to the lesser value on the thunderforged)
2) Possibly didn't change the reforge and extra stats just flat out got wasted and again, the absolute value > relative value.