Its not clear why
Acetaminophen is thought to have some effect on local transmitter that mediate pain/sensations. An antihistamine like pyrilamine also works in a kind of similar fashion (on different transmitters). The two together may result in a slightly different effect/side effects that may explain the above. Other than that no other obvious explanation comes to mind. Be careful not to consume excessive acetminophen as in excess it can cause liver and kidney damage, and even death