You've done your calculations as though all the money you lodge will be in the account for a full year and will earn a year's interest, but this is not the case.
For simplicity, say you open an account on Jan 1st 2006.
By 31st Dec 2006, the initial €20 and the January payment of €20 (both lodged on 1st Jan) will have earned a full year's interest, but the €20 lodged on 1st July will only have earned interest for 6 months, and the €20 lodged on 1st Dec will only have earned interest for one month.