Why are 1967 pennies worth more than 1966 pennies?

Because there are no 1967 pennies. Decimal currency started in 1966.

Actually, 1966 pennies should be worth more than 1967 pennies as a 1967 penny would be a fake.

CorrectionThis question was posted in US Coins so it refers to US 1-cent pieces by their slang name.

It's actually a rather lame joke. The answer is (ba DING!) "1,967 pennies are worth $19.67 which is one cent more than $19.66"