The term "433MHz" refers not to a frequency, but to a band. That is, a range of frequencies, officially defined as 433.05MHz to 434.79MHz. You are free to use any frequency within that range you choose for your carrier frequency. The manufacturers of the garage door opener have chosen one frequency, your TX/RX are tuned to another frequency.
If you want to receive the frequency your opener transmits at then you will need a receiver tuned to that exact frequency. If you want your garage door to receive your signals then you will need a transmitter tuned to the correct exact frequency.
You can think of it as a normal radio. "433MHz" is akin to "The FM band". Within that band are a number of stations, all with their own frequency. Your TX/RX pair are tuned to BBC Radio 2, but the garage door is working on Classic FM (replace with two radio stations that mean something to you...)