That’s because SMS has a 1120 bit message capacity and uses one of two encoding mechanisms.

GSM-7 uses 7 bits per character and a correspondingly limited character set that allows for 1120/7 = 160 characters.

UCS-2/UTF-16 uses 16 bits per character for a total of 1120/16 = 70 characters per message.

It's not weird because it's not a 160 limit in the sense of characters. Non-English language people are I guess more aware of this -- using a character not in the standard English a-z often counts as two characters.