Why are some license plates stickers issued for 2013 already?

Because people purchase 2-3-4-5 year (depends on state) registration stickers.

So next time you register your car, ask for a longer registration period, and the sticker will have the year it expires on it.

For example, if I went today (12-30-2011) to the DMV and purchased 2 years worth of registration in backwards Idaho; then my lic. sticker would have "2013" on it, with the month of december printed on the side of the year.