Patek has made a point of recommending regular service every 3 years and that starting point is generally from the date the AD receives it. However, the distributor may have had the watch in its inventory for quite some time before shipping it to the dealer. Then the dealer may sit on the watch ...well, for years.
So when a client buys the watch, who knows, it could very well be 5 years or older. The recommendation by Patek is for the dealer to monitor their inventory and send watches older than 3 years in for service. Really, how likely is that to happen? Inventory being gone for 3-5 months, maybe longer? After all, time is money and dealers are under quite a bit of pressure as it is.
Even though it would be quite simple for the AD to notify Patek when a watch is sold and the clock starts at that point, the watch could conceivably be 5-10 years old before it's sold.
This doesn't even address issues where a watch is sold, returned .. sold again, returned again, etc. Some stores have policies where clients can wear a watch for a period of time and return it for exchange. And this may happen over and over again. Then, how new is it? Now what about the warranty? I never really had issues with this sort of thing because I controlled it but now most dealers are pretty large operations where things may not be as tightly managed.
I do agree with your thought in that at least everyone would know the conditions of the warranty and would be far less confusing regardless of the number of owners within the 2 years after it is sold.
Your last statement I feel is absolutely correct, but the real world of money gets involved and results in a lot of what you said. Do you love a dealer who does it right and sells $500,000 per year or the dealer who doesn't always do it right and sells $3,000,000 per year? Take a guess at who is loved more.