As you already mentioned, maybe the winters caused the fuel dilution.
Winter -> Cold Weather -> Takes longer to reach operating temp
I believe that if your oil doesn't get "hot", the fuel stays in the oil instead of being "burnt" off. (Unfortunately, I don't really know the chemistry between fuel and oil... so I can't really back this up!)
This is why short trips, excessive idling and cold weather can cause fuel dilution.
As for why thinner oils help fuel dilution, I never found a concrete answer. Do thinner oils heat up faster? I also remember reading a bitog thread that thicker oils tolerate
fuel dilution better than thinner oils. And viscosity doesn't affect an oil's affinity (or lack of) towards fuel. (IIRC, from Molakule).
Too many questions, so little knowledge.