That thinking has gotten a lot of people in trouble. The manufacturer of a prebuilt system will buy a mobo that is physically capable of using any CPU for that socket, but they still have to write a bios for the system. A lot of times, they only write in the CPU they are putting in to save money on programming cost.
You see a similar phenomenon with homebuilt systems when you are trying to put a newer CPU in an older mobo. Even if they use the same socket, the old bios was written before the new CPU existed, and won't work without a new flashed bios if one exist.