Because the interior drawer is recessed, the length of the drawer box is shorter than the cabinet interior.
Please see the attached profile image.
The drawer box is @ 40% shorter than the cabinet, which means the slides are also shorter.
Full extension slides actually only “open” the drawer @ 80% past the cabinet overhang.
So full depth, less 40% less another 20% and the result is a drawer which appears to extend only @ 45-50%.