My example is pretty simple. An antenna does not radiate only 900MHz. It radiates a distribution, centered around 900MHz. The fall-off is rapid, but there is non-zero power at higher frequencies, and there can also be harmonics. It's just an example, but the point is there could be some hidden effect we're not considering or aware of. It would hardly be the first time we thought we had it all figured out, and whoa hey there, we didn't expect that to happen.