Why does a direct-vent appliance, 2 pipe (over 50K BTU) have a 12” clearance termination from windows, etc., and a non-direct (one pipe) have a 4’ clearance from the same?

NFPA 12.9.2 A mechanical draft venting system of other than direct-vent
type shall terminate at least 4 ft (1.2 m) below, 4 ft (1.2 m) horizontally
from, or 1 ft (300 mm) above any door, operable window, or
gravity air inlet into any building. The bottom of the vent terminal
shall be located at least 12 in. (300 mm) finished above grade.
[ROP 54/Z223.1-2]
12.9.3 The vent terminal of a direct-vent appliance with … an input over 50,000 Btu/hr (14.7 kW)
shall be at least a 12 in. (300 mm) vent termination clearance. The
bottom of the vent terminal and the air intake shall be located at
least 12 in. (300 mm) above finished grade. [ROP 54/Z223.1-2]