Spiders on surface??

Well, I noticed in the winter season (I am playing in) I found some spiders on the surface?? Found this 3 times. One time at a cliff one time on a mountain and one time just on the plains.

Is this a bug or a feature? (I really like it

could be... Still the npcc's cannot spawn in buildings whit 4/5 blocks high, then they teleport to roof. also sometimes they walk on a table and then fly trough the roof. maybe that happend to the spider xd.