I am not sure, but I think it is one of the following:
1-other bugs are also coming in holes and staying between the floor of the upper level and the drop ceiling of the lower level. All of the spiders we see in the lower level come out of the gaps in the drop ceiling. The prior owner didn't hang it properly (we are going to redo it) and we actually see the spiders sitting in the gaps between the ceiling tiles and the brackets. I can't look up there because the prior owner only left an inch or so space and I can't get the tiles out without starting from one end and taking them all out.
2-the spiders travel inside to get warm and go outside to gets food.
I am going to screen the holes in the soffet and plug the other cracks around the house and also redo the drop ceiling without any cracks. Hopefully this helps - if not I will contract with a exterminator to spray the exterior every four months.