DVD players leave me confused?

I had assumed some disks were warped because I played them on 2 separate DVD players and two computers and not at one time did I get a clear play out of them...now when I was testing an old small apex dvd player I decided what the heck and tried it and wouldn't you know it? They all played!!! This has left me confused! Why would only some DVD players play a disk and not others? These are movie disks by the way not burned or illegal copies....I ordered them online and they are NTS format so they are not foreign....I'm just perplexed!