It sounds okay if you indeed did get warranty replacement (unlike an exchange in the first ten days). Under warranty, at least here in the US, the manufacturer can either repair or replace at their descretion so you may have just received a different display also returned on warranty and repaired. Yours will at some point probably also go out to someone else when it's repaired as well. Usually, in these cases, the serial number will have some sort of indication that it's a refurb or repaired unit but i'm not sure if Apple does that for warranty work. I really don't think it's a store demo though.