Aerodynamic drag.
Drag alone is not enough to determine impact speed. Weight is important, too.
The heavy piece hits at 99 mi/h.
They show a 59 pound piece falling at the same speed as the heavy one, an 11 pound piece falling at 59, a 2 pound piece at 40, a 4 pound piece at 30, and 11 pound piece at 81, and a 1 pound piece at 47.
There are 5 lighter types of pieces falling faster than 99.
So, some light pieces fall faster, some slower than the heaviest piece.
Details of the UARS satellite debris to hit the Earth
(1) HGA gimbal and reten. (Titanium) 59 pounds impacting at 99 miles per hour
(4) Forward bulkhead fitting (Titanium) 55 pounds impacting at 177 miles per hour
(1) SSPP gimball (Titanium) 134 pounds impacting at 130 miles per hour
(1) SSPP structure (AL-2024-T8) 348 pounds impacting at 99 miles per hour
(4) MMS fuel tanks (Titanium) 11 pounds impacting at 59 miles per hour
(3) MMS MPS batteries (SSteel 304L) 101 pounds impacting at 146 miles per hour
(4) Reaction wheel rims (SSteel 304L) 4 pounds impacting at 240 miles per hour
(1) FSS housing (Beryllium) 7 pounds impacting at 175 miles per hour
(2) FHST bracket (Beryllium) 2 pounds impacting at 40 miles per hour
(2) G.F. abutment plate (Titanium) 4 pounds impacting at 30 miles per hour
(2) G.F. base plate (Titanium) 11 pounds impacting at 81 miles per hour
(1) G.F. extension (Titanium) 1 pound impacting at 47 miles per hour