I'm a high school physics student studying about resonance.
Resonance happens as a result of a forced oscillation. If there is reflection within the system when resonance happens, we end up with a standing wave. If the object forcing another object into oscillation isn't doing so at the forced-upon object's natural frequency, you'll get a weird periodic wave pattern that is overlapping. (I think it's called beats.)
Now, let's do a thought experiment where we force an object to oscillate but not at its natural frequency. At what frequency will the forced upon object oscillate? In this case, I am not concerned about reflection within the system whatsoever. I'm purely concerned about the frequency at which the forced upon object is gonna oscillate. I was always taught that the frequency of oscillation depends upon the source, but by this logic we should just get a smooth clean standing wave and we wouldn't need both the forcing and the forced-upon object to be at the same natural frequency.