How did people make up the lb. abbreviation for pounds?

It's no accident that the word pound represents both a measurement of weight and the currency of England. One pound sterling was originally worth the same as one Troy pound of sterling silver. So, at one time, 10 pounds (currency) was actually the equivalent of 10 pounds (the weight) of silver.

The basic Roman unit of weight was the libra pondo; libra means "scales or balances," and pondo comes from pendere, meaning "to weigh." Pondo became pound, and libra became the abbreviation lb., reflecting the medieval custom of keeping accounts in Latin.

Incidentally, this also explains why the zodiac sign for Libra is a set of scales.