When your phone is in your pocket, there are two possible states: it is eith

When your phone is in your pocket, there are two possible states: it is either vibrating or not. Meanwhile, you also have two possible states of mind: the judgment that the phone is vibrating, or that it isn’ t.
Ideally, you match the four states, correctly. However, sometimes your brain decides that the consequences of missing a call are more serious than a "false alarm". Therefore, you become more sensitive to vibrations so that you don’ t miss anything.
This is just like the fire alarm in your home—it can be annoying when it goes off in response to just a tiny bit of smoke. But that’ s a small price to pay compared to a fire really breaking out because the alarm fails to alert you.