Well theres different reasons for it, software and hardware bugs, timeframes, standards added or changes. for products like wireless routers especially, if there's some hacked think like WEP they'll often update it.
its a good thing usually, but places will often chuck it out earlier thinking they'll patch it after launch, but this is usually a direct response to the demands of the customers who want it fast and cheap.
Exactly.
And when it comes to products that use a "standard" protocol there's one more twist to put on it. The explanation I like best is one I heard from a very senior engineer at my old job: it's what it means to be a cable whore.
What he meant by that is, even if your product does everything exactly as it's supposed to per the spec, not every other third party product your products connects with will. And that means sometimes you actually have to do things outside what's required by spec to make it work. If your company is the gorilla in the particular market, and your product gets out early enough, you can actually often become the defacto standard, even if it's not 100% in compliance with the spec. The company I'm at now is classic example of this. They put out a chip for a SATA connection and it was dead on exactly what the spec says. However, almost everybody else product we were trying to connect to had it wrong. Our customers agreed the other companies had it wrong. Even plug fests showed the other companies had it wrong. However, in the interest of expediting our customers products (we just made the chips and firmware, not complete end product), we made the necessary changes to get our product to work with the other POSes.
The more off the wall brand and cheap POS product you go with, the more likely you'll run into this sort of thing. Wireless hasn't quite reached that commodity state (IMHO) like say a 10/100 MAC has so expect to see issues.