Our brains are really good at filtering the information from our nerves. You don’t notice your clothing constantly rubbing against you, mild breezes etc.
However, if you have something similar to an insect landing on you, your attention will immediately be drawn.
Phones are the same. it normally takes more than a month of phone ownership to detect “phantom vibrations”. After you have taught your brain “vibrations from my pocket are important”, then your body will instantly draw your attention to anything similar, even if it’s vibration from music, a muscle contraction, clothing catching on your skin.
Because, you know, it could be a killer mosquito and best not to take chances.
tl;dr your brain is trying to interpret information from lots of “dumb” nerves to work out what is going on and sometimes confuses vibration for other similarly spaced stimulation.
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