Your oil alone does not have enough time to really heat up and thin out. so you have massive oil drag especially in the winter. Your very short trips are basically considered hard use

. The highway trips do allow the condensation in the oil to boil off. All told your reported milage sounds about right for the uses described.

Using the block heater might help enough to offset some fuel cost. It also would make engine wear less for those extremely short trips. Most engine wear is considered to occur with cold engines is the general belief.