Dr. Deb Bennet has a wonderful article on this. http://www.equinestudies.org/ranger_..._2008_pdf1.pdf
On page 8 of the article you will see where she says, "These do not finally fuse until the horse is atleast 5 1/2 years old (and this figure applies to s small-sized, scrubby, range-raised mare. The taller your horse and the longer its neck, the later the last fusion will occur. And for a male add six months. So for example, a 17 hand TB, SB or WB may not be fully mature until his 8th year."
So I would not be surprised if your mare grew a bit more. It may not be much, but she is still maturing if she is that tall. Also, muscling out will also increase the height of the horse by a bit above and beyond just the skeletal structure.