Have you tested the voltage at the sockets? Use one of those screw-in
socket-to-outlet adapters.

Any reason why one of the bulbs might get hotter?

Maybe only 'that' bulb touches the fixture, or the globe.

Maybe 'that' bulb doesn't screw in so tight and is more prone to
vibrations.

For grins, try leaving the globe off for awhile and see if there's a
difference.

Dave