The only grand difference between "Standard" Shin/Instep pads and "Grappling" Shin/Instep pads is that the grappling variation are put together with the grappling in mind. Because of that, they are designed to be less bulky and more secure when rolling.
Most standard Shin/Instep pads secure themselves with 2 straps, with a loop for the bottom of your foot, just like this one here
. They are also very bulky. The extra heft does not translate well to doing ground work (from top or bottom) and you cannot grab a hold of an opponent's leg (or your opponent grabbing your leg) with the assurance that it will stay in place, or even on your leg. For striking, it's perfect. For grappling, not so much so.
The grappling Shin/Instep pads are less bulky and usually have a full sock or "boot" of material that runs the length of your calf to hold the pad in place securely. This style here
is a prime example. With this, the pad offers functional protection for sparring, but it will not shift around or come off if someone grabs a hold of your leg.
That's the difference in a nutshell.