I think it's just the heat retained in the bulb from the light filtering, but Wood's Glass and Quartz are UV transmissive and hard glasses (~borosilicate & Soda Lime Glasses) are UV absorptive, which is way standard hard glass outter envelopes block UV at least somewhat in standard bulb envelopes.

I'm sure the filaments run hotter on BLB bulbs the whole bulb seems to.

Running parallel BLB bulbs and gotta change allot.

Gotta run. Finish this one next time.

Take care, Sal.