While there are numerous reasons that a certain room maybe cooler or hotter than others - the garage can be a big factor. In many cases the air sealing and insulation work in that area is inadequate or completely non existent.
The only real way of knowing what the issue is, is to either do some exploratory work yourself or bring in a competent energy auditor.
For more information:
Read "What are the best ways to green my garage?" a Q&A answered by Dennis Cheslik.