byhalia, any time you adjust your chain, whenever you loosen the axle nut, if you loosen it enough so that all "flex" (for a lack of a better word) in the swing-arm is relaxed, (the swing-arm is spread as far apart as it goes) then you adjust the chain, when you tight the axle nut back up, the swing-arm comes back together effectually "lengthening" the swing arm. this in turn makes the chain tighter than when you first adjusted it. what you should do is loosen the axle nut just enough to allow freedom of movement of the back wheel along it's adjustments without allowing the swing-arm to spread too much. this will keep your adjustment closer after tightening. a little practice will take care of this. just try to adjust the chain a little slack to what you want, then tighten the axle nut and check it again. should be just a tad tighter than your original adjustments. good luck
hope this helps.
mike