The rails are a little different size as a design choice and, it also bought me a little needed room to fit the 2 file drawers.
The under mount glides (Blum) are typically mounted to the sides. The middle set in this cabinet will be unsupported on the bottom but, I'm not expecting anywhere near the weight load for these glides. I chose the under mount for drawer size considerations. With side glides, you lose 1" to the width of the drawer and keep them at full depth. With under mount glides, you lose 1/2" to the depth of the drawer but, keep the full width. And, the Blums have a lot more adjustment options as well as they just function better. Over all, they're just a better glide but, their price point goes right along with being a better glide..
I'm not sure where you saw AM ply? I reread my last post but, I've obviously missed something somewhere.. Not that that is anything new...lol