That assumes that notes spent at a store are kept and never reused; that's simply not true. If you buy a €5 coffee with a €5 note and then later in the day someone buys a €3 muffin with a €10 note, your €5 note is given to them as change. Repeat that process in a town or city and that cash gets pretty shuffled quite quickly.
Your scenario doesn't also take into account till "float" where a portion of cash is kept in the till/store/elsewhere to seed the till the next morning for the first customers of the day, should they need change for their purchases.
I get what you're saying. However, their entire business model is predicated on them being impartial. If it turned out that they were biased, their business would collapse.
Look up the "no poo" movement. It's been over 5 years since I washed my hair and it's the best it's ever been.