The spacers were beefed up to help reduce potential deflection of the wheels/flexure of the bolt shaft. But once they were beefed up, it rapidly became apparent that the new larger 10mm eccentrics would hit the extrusions when using the mini-V wheels. Thus the newer version of the old 8mm eccentrics was created. The newer version has the added flared portion where the spacer meets the face of the plate to provide additional stability. As for why they carry both, I don't know, it's not my department. Probably just to maintain continuity for those already using the 10mm eccentrics. The newer 8mm eccentrics may also be more expensive due to much more limited production. But that's nothing more than speculation.
@Rick 2.0 : With the C-Beam actuators the 8mm versions became more popular so it's about 50/50 for 8 and 10mm (wrench size) spacers. @Adam Filipowicz : As a quick tip: to make a build sturdier, use 10*5*1mm shims on the opposite side of the gantry plate, so, one side the eccentric spacer and the other side the shim. Now the full size wheel spacers are not in stock there won't be any problem ordering the mini wheel spacers. A 3mm aluminum plate will bent before the spacer will, even a 6.35 1/4" aluminum plate will probably bent before the spacer will .
