Not enough information to go on but here could be a couple of reasons.

It may be an oversight or type O. It could be below grade like a basement. Some states do not allow you to include a bedroom below grade in the room count. It may not have a closet. Again, some states do not allow you to list it as a bedroom without a closet. And lastly, it could be a tandem room where you have to go through one room to get to the other, no entrance of it's own. Some people don't like to list houses with tandem rooms as a 3 bedroom because people who are looking for a true 3 bedroom would be disapointed when the get to the house. This way if it's listed as a 2 bedroom when they get there is becomes a bonus rather than a negative.