i've a simple question about ADC conversion to voltage

let's consider an atmega8

set ADC accurancy to 10bit, so that my ADC data register will return values from 0x0000 to 0x03FF i.e. 0 to 1023, i will have 1024 values

if i would like to get a voltage for a pin, i should read ADC value for the analog pin, solve this formula

Vin = ADC * Vref / CONVVAL

where ADC is the ADC value read on the pin, Vref is the ADC reference voltage

reading atmega datasheet (ADC Conversion Result pag 199) CONVVAL is 1024

now my question, ADC can have a minimum value of 0 (0x0000), and a maximum value of 1023 (2^10-1, 0x03FF), i do not understand why CONVVAL is 1024 from datasheed and not 1023, also i've found some source code using 1023 and other 1024.

anyone can clarify this to me?

thanks ;)