Someone with proper knowledge will be along shortly but till then I'll pitch in.

The ones with the grip at the front of the action are called mid grips and that's how the XP100 is. Balances better that way.

The ones with the grip at the back are either a conversion so that the owner can run a different trigger or, more likely, a XP100R. The XP100R was a rear grip magazine fed pistol. The rear grip was needed so that there was somewhere for the magazine to go since on the mid grip the trigger mech runs where the mag would go.