Is this just the way they are measured or is there a real difference?
There are legit differences, depending upon the brand/manufacturer of the lifters.
Do I need to buy a test lifter first when it comes to a race engine?
Not a bad idea, especially if you're trying to fit the lifters to lifter bushings, etc. The recommended spec, IIRC, for roller lifters in iron or aluminum blocks is .002", which is a crap shoot if one brand measures .9031" and another measures .9038"... which is exactly what I found myself.
FWIW, I went w/ the .9038" brand, but my lifter bores are still too loose due to previous wear w/ the bronze bushings.
