The difference is, when you switch your cellphone off, it does not cut the power immediately, but it actually talks to the BTS "I am about to being switched off" (and then cuts the power, mostly). When somebody calls your number, the BTS already knows you have switched off your phone, whereas if you move out of reach, the BTS attempts to reach the phone and if fails, it assumes you are out of reach.

You can experiment with this.
Try removing the battery from your phone and call the phone, it should behave as if you would move out of reach of BTS.

Actually, this has consequences to your money. When you are in roaming (abrouad) and if you have redirected calls when unreachable, when you don't switch your phone off correctly (and move out of reach of BTS or drain off your battery or similar), any incoming call gets redirected from the country where you are so you will pay the cost of roaming plus the redirection. If you switch it off correctly, the redirection won't go through the foreign country so you pay only the cost of redirection (if any).

Jan Waclawek