WHY IS THAT?
It has to do with percussive overtones when the reed slaps against the mouthpiece body every time it vibrates. Two materials are involved, that of the reed and that of the mouthpiece. Different material combinations create different percussive overtones, which flavor the base tone of the pulsing air. That's why different mouthpieces used on the same body create different sound qualities when playing the same note, vibrating at the same rate.
Plastic reed against plastic mouthpiece is very tinny. Silicone rubber coating the plastic reed material helps; silicone against plastic. Silicone reed to bamboo mouthpiece is better. Silicone reed to silicone mouthpiece is best.
I have plastic drumsticks with rubber heads. Hit the sticks together and there is a sharp sound. Hit the rubber heads together and there is a soft dull thud.
Since there are fewer percussive overtones, the base frequency sound of the reed vibration rate is cleaner, and less tinny.
Be sure and hear the sound sample in the final step.