5% of physical currency is a HUGE amount though. In any given week I expect I go through about 10-15 banknotes (eg changing a £20 into a £10 and a £10 into a £5 is 3 banknotes).
I've only ever seen
one forged note in my life. Now it could be argued that I'd seen more, but they were so professionally done that I didn't realise.
I doubt the actual amount of real forged currency floating around the economy is more than 0.001% though. Obviously I've picked that figure off the top of my head, but then that's the most fun form of statistics