imo, everything about archery is personally subjective, and that huge variability can mean if you shot my *excellent* bow and arrow, it might just be a *dog* to you. i take bow and arrow marketing rap with a shaker of salt 'cause the rubber meets the road when *I* try it all out.
if every *great, fast, wonderful, killer, fabulous, superb* bow was truly as advertised, there shouldn't be many used bow ads in the classifieds forum, eh? :D
there are two arrow spines to consider: static and dynamic. static is ballpark, dynamic is what counts and what matters for arrow tuning.
arrow spine, static or dynamic, doesn't change of its own volition. changing arrow spine requires a physical property alteration, such as in mass weight or balance. shooting an arrow out of a bow doesn't change either its static or dynamic spine. this is basic wysiwyg stuff.
static arrow spine is the applied weight deflection reading you see on a spine meter. multi directional arrow shafting (carbon, aluminum) spine is very close to 360 degrees around the shaft.
dynamic arrow spine begins upon arrow release, and goes through some measure of paradox that's defined by the release type and its efficiency (the trigger), the bowstring (the transmission) and the first physical obstruction the arrow encounters other than the ambient atmosphere (the rest, the arrow plate, the riser). there are so many other factors to include, such as arrow shaft and nock straightness, arrow balance, fletching type & size & location, nock point location and type, and many others including what launches the arrow, human or machine.
if you paper tune, the backstop doesn't matter at all since yer punching holes in suspended paper that's a good distance from the butt.
if you target tune, the butt must be made of a multi directional material, such as ethafoam (polyethylene). stuffed bag and layered foam butts are unidirectional and will give false tuning indicators.