Is that wear level common? I don't know what kind of algorithm you use but is it normal if I just got the tablet one month ago to be at 8%?
Wear level is the difference in percentage between Full charges capacity and Designed capacity.
So in your case: (38991/42002 * 100) -100, and rounded.
Wear level is estimated. Full charged capacity varies every recharge, so it is normal to see the wear level fluctuate several percentage points. One day it may be 7%, and another 4%. But, it won't be (unless something is wrong), 3% and 70% the other day. It doesn't fluctuate like that. Just a few points up and down.
On my side I don't have your problem. My cycle count is at 48, I got it in early February, wear level right now is 5% (but at times I see is 7%). My device always recharge at 100% just fine. The first I got it, I think the wear level was at 4% (which is fine, because, like I said, you can't charge Lithium-ion batteries at a true 100% every time, it will fluctuate.
So honestly, I don't know what is wrong. If the wear level continuous to increase, I would be concerned (faulty battery or recharge circuit).
If you can return your Surface Pro 2, I would do this now, and take no chance. Else than expect a warranty replacement.
I do know it is NORMAL for the recharge level to reach 100% if it was like 90% or 95% or something, which is by designed. Because pushing lithium-ion battery to it's max, it considered dangerous. But if the battery is like 40-50% then for sure it should recharge at 100% just fine.