I love Zillow as it is a tool that would have cost many hundreds 10 years ago. Now we get the info for free without contributing to the real estate appraisal business.
As to the problems, Zillow gets numbers by looking at recent sales and at tax values. As seasonal trends go, with houses usually sellling for less in the winter than the summer you can expect to see a lower value in the winter. I wouldn't worry too much as you aren't trying to unload it. However, it does help you know when you should sell it in the future.