There are three possibilities. Well, maybe four.
Simplest: The vehicle may have been parked at an angle when you checked it.
Or: If the fluid is hot, it will expand and cause the level to rise.
Or: It may have been over-filled by the prior owner.
Worst Case: You may be leaking fluid between your transmission and transfer case. Check the trans fluid level. If it's ok, then you're probably safe.