Small block pushrods with hydraulic cam.

Companies use different ways for listing length.
Comp Cams listings uses the Gauge Length Method.
Different method is most likely what your seeing.
http://www.cpgnation.com/forum/threads/comp-cams-pushrod-tech.2408/

On a ball/cup I use a length checking pushrod (old pushrod cut and 10/32 threaded rod to adjust proper length) Remove from mockup and place a 5/16" ball in the cup. Measure total length and subtract 5/16". That number will give you the length from the bottom of the cup to the end of the ball. Smith brothers can use that number to make custom pushrods for you.

Deck height, lifter preload, head gasket thickness, head milling in the past, and rocker arm all effect proper pushrod length. Pushrods are the last item to order when building a motor.