A few reasons I can think of.
A. The quality or type of the device that took the video or tech that uploaded the video.
B. The first two videos are on the lawn and the machine is surrounded by grass. The third video is on concrete. Reverberation of sound is different depending on the materials. Just a thought.
C. Do some of the machines have a baffle or shredder? That might also change the tone.
D. Possible the machines have different engines, mufflers, or the condition of them varies.
A. The quality or type of the device that took the video or tech that uploaded the video.
B. The first two videos are on the lawn and the machine is surrounded by grass. The third video is on concrete. Reverberation of sound is different depending on the materials. Just a thought.
C. Do some of the machines have a baffle or shredder? That might also change the tone.
D. Possible the machines have different engines, mufflers, or the condition of them varies.