What are the lateral limits of Class D airspace in the USA?
JO 7400.2L, Procedures for Handling Airspace Matters, describes how the size of Class D airspace is determined. Per paragraph 17−2−5. DETERMINING CLASS D AREA
SIZE:

The size of a Class D area, and any necessary extensions, is determined 
by the use of a 200 feet per NM climb gradient and information obtained 
from the person responsible for developing instrument procedures (see 
FIG 17−2−1).


The figure lays out the formula for determining the radius and gives an example. In short, it depends on the distance from the airport reference point (ARP) or geographic position (GP) to the end of the outermost runway, and the terrain surrounding the airport. This is why the difference varies at different airports.