The rear two plates of the shell on the underside of my tortoise apear to be flexible, is this normal?

I was looking at the underside shell of my tortoise, she is a 50 year old Greek Spur-Thigh. The rear two plates on the bottom of her shell, inbetween her hind legs, althoughhard and rigid themselves, the join between them and the rest of her underbelly seems to be flexible so that the joint bends is the plates are pressed. Is this normal?