Well that is true, but I have never had it cause a problem. If it is tilting very slightly to the rear, that would be a chain a little on the long side, to the front would be a chain slightly too short. As you say you want to choose the length for which one more or less link makes the two pulleys point the other way. Depending upon your philosophy you can choose which one you prefer. The same thing happens with all chain fitting methods. For example in the largest-largest+1 method, the fit isn't exact either in most cases, but if you insist the chain must come together and have no separation, you will almost always be a little long. You don't allow it to be a little short. So with Shimano you could achieve the same thing by always having the pulleys either perfectly vertical OR slightly pointing down to the rear. No real difference there.