How come? It seems it is very analogous to water in a pipe. Let's say I have a 30 m long garden hose. The water in it moves (in the order of magnitude of) 10 m/s in it. If the water is turned off, and I hold my thumb at the end of the hose and turn the tap on, I feel the water pressure almost immediately (the speed of mechanical waves in water). So it is obvious that there are two speeds. One is the speed of propagation and one is the speed of the medium, and they are way different.
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How come? It seems it is very analogous to water in a pipe. Let's say I have a 30 m long garden hose. The water in it moves (in the order of magnitude of) 10 m/s in it. If the water is turned off, and I hold my thumb at the end of the hose and turn the tap on, I feel the water pressure almost immediately (the speed of mechanical waves in water). So it is obvious that there are two speeds. One is the speed of propagation and one is the speed of the medium, and they are way different.