It's because of the thickness and the type of metal, as well as the signal type.

If your tin were made of copper or aluminium it's very possible AM would have been reduced enough to be unusable, but the tin is made of a less conductive alloy than either of those, so it has a harder time keeping out the waves.

The higher the wave frequency is, the thinner the metal sheet can be to keep it out/reflect it.

Apart from that, an AM signal is a wave that is stronger and weaker in the frequency of the sound. AM stands for Amplitude Modulated/Modulation. So the signal gets weaker and stronger to identify the audio wave. Because that system is very sensitive to the signal strength to supply audio, there's an auto-gain control on it, that tries to amplify it.

So there is some signal left, because the tin isn't a great conductor and just about thin enough to let some AM frequencies through, and then the Auto-Gain tries to make something of it by amplifying it till it makes some semblance of the original audio.