Just compared a couple of spares I have and they are very slightly different. The solid plastic base part appears about the same, the foam pad on the 28 is slightly wider, a little higher and is longer at the front.

You can interchange them and you would struggle to tell the difference to be honest but the 21 pad is waisted in more on the sides, the 28 sides are almost straight. If that makes sense? In fact you might even have a 28 pad but hard to tell from the angle of your pictures.

Without wishing to insult your intelligence have you hooked the hooks on the base under the hooks on the subframe? It is sitting up a lot.