My guess is that it's because there are five banks, which correspond to the five physical buttons marked '1', '2', '3', '4' and '5'. E.g. pressing '2' takes you to the patches which start with '2', namely 20-29. But if there were patches 0-9, you'd need a physical button marked '0'.
The tuner's cool as it uses the green & red LEDs underneath the display.