Its a mystery to me where Tabitha's room was located at the top of the stairs. At different times it seemed to be in different locations

Tabitha had two different rooms. The first was the nursery that she was in as a baby, the second room was a blocked off version of Sam and Darrin's Master bedroom. It would be interesting to know that in one episode, Sam and Darrin's bedroom (the second room) was used as the Tate's bedroom. All it was is same set, different purpose.