Polo thinks this shows Perl is quite stupid. I think it shows she is quite clever, though missing a few basic facts.
My reasoning is that Perl knows that the world is different if you go out of a different door. The scenery is different, the smells are different, and sometimes it is sunnier if you go out of the South-fecing back door, and shadier outside the North-facing front door. At night, it is sometimes light outside the back door (if the rear light is on) but dark outside the front. Sometimes it *is* wet on only one half of the house (if someone is cleaning the windows with the hosepipe).
Given this, it seems quite sensible to not assume that weather will necessarily be consistent around the house, when so many other factors are variable, but to go and check.