One possibility where this can happen is if someone has bled the brakes without inserting a bleeding block between the pistons. With new pads and disk between the pistons, in theory you could bleed the system without the bleeding block. However, if you bleed the system with old pads and disk between the pistons, the system becomes overfilled with brake fluid, and you are never able to install a new set of pads anymore, unless you untighten the bleed port screw on the levers, and push the pistons in with the bleed port screw removed.
Warning: pushing the pistons in with the bleed port screw removed will be messy, brake fluid will leak out from the levers.