I'm sorry, I'm not sure what you are referring to. Are you getting "A" and "B" from the hub on the math behind bending conduit? These numbers are associated with the amount of offset and the straight line distance it takes to accomplish the offset (not the length of conduit it takes). There really is no term to refer to that difference that I am aware of, either mathematically or in the electrical field. The difference between B and C, however, is termed the "shrinkage" of the conduit; the amount the straight line distance will decrease when bending offsets or saddles.
If the question relates to the hub on bending a 90, I am a loss to answer it. A little more explanation, please - I will do what I can to help you.