You should still measure the sockets where you say the bulbs last longer.
Even if your voltage is correct at this location, if it is lower at other locations, bulbs will last longer there.
No, I don't especially think that is the situation, but when one does testing, he should test and record everything that could be relevant. Later on, he can figure out what is important and what isn't. Although he likely won't figure that out until he figures out the problem. That's why he needs all this info.