Without analysing more (pics) of your chair, I suspect the armrests may have been cut off at the back, possibly after one broke. The backrest needs something for support.... I suspect, more so, than what your chair is showing.

Your chair may have had longer armrests, for supporting the backrest, as this chair:
http://antique-4-sale.com/images/mot...t---2380-3.jpg

Sonny