As you might have seen, that statistic was suggested quite some time ago.
Average Cards Left statistic, is very situational and has lots of variables. I totally agree with you in it. I myself fill up that value between 1 and 3 very often and having eternity i know i last for more than that.
About cards won, it's just as it says, cards won. We noticed some people keep track on that on the forums, just as general statistics.
None of these values influences how good or bad the deck is, and cards left on deck may also not mean that deck is too small or too big for the reasons you and i have remembered, possibly others. Hence, both these values are optional. If you can't be bothered to insert them, don't, i guess anyone can say you didn't by checking they're not really right (if inserted couple of them when started), or if at zero.
Also if anyone wants to pay attention to ACL on deck, we can assume for now they know the deck which has been the case, they can tell its possibly not really important, mainly when having eternity in it.
Then again, you don't have to fill those values.
If you want for some reason, i can delete that data from your battles, leaving the rest intact, making it seem you never inserted it.
Let us know about any suggestions you might have ;p