i am an oem system builder and there are many reasons for this

if the cmos battery is low and it reports the wrong date or time , a lot of the time someone steals the product id number from the side of the computer or a using a pre generated number from a keygen, and i have seen some oem copies fror sale from ebay that are fake and actually look like the real thing, the only way to guarentee a geniune copy is to get it from a reputable computer dealer. most of these cases all you need to do is ring microsoft and they will rectify the situation if youve been sold a fake copy they will sort out a new copy if you can prove proof of purchase

in the uk there are only 3 geniune distributers of oem software from microsoft so they do know exactly how many go out and to whom

i have had a situation where a customer has had a system reinstalled with theyre geniune copy and it did come up with non geniune version because somone had already stole the key, but at the end of the day people have been using copied windows no for over 10 years and had a good run and £60 with a years free support for a geniune copy isnt expensive, there is always linux