A slap carries a lot of speed but little weight behind it.
Your nerve endings on the surface report a whole *lot* of sudden compression, as they would from getting struck with an utterly devastating punch.
The blow doesn’t penetrate and cause much damage, but they’ve already sounded the alarm.
Also, slaps tend to cover a wide area, which sets off a whole lot of nerve endings at once. Your skin thinks someone’s swung a bowling ball at you, even though the underlying tissue begs to differ.