If you enter an expense of €10 and three people participate, the costs per person are €3.33. But 3x €3.33 is €9.99 and thus your are missing €0,01. Because we want all balances to be exactly correct from an accountancy point of view, one of the three participants needs to pay €3.34. The system chooses this person randomly, only the payer of the expense never has to pay this extra cent.