from top of tube measuring down to top of hole where triangle is inserted is 17 5/8"
OK, so +3/16" gets you to center of axle...17-13/16" (approx)
Plus half of a wheel diameter, 4" gets you to 21-13/16 "
This all seems close enough to be correct allowing for different people measuring, end caps and depth of leg seating.
If the OP is off a full 2" he has got to have the wrong rear legs, if I am tracking this correctly.