ATmega16 Timerr Fast PWM
The OCR0 register controls the duty cycle. Since it's an 8-bit microcontroller, it deals (internally, at least) with 8-bit values. As such, the register has a range of 0 (0% duty cycle) to 255 (100% duty cycle). This is simply a design choice that allows for simple translation of internal values to real values, with a resolution high enough for general purpose.

If you do the math on a range of 256, setting it to 64 gives you 25%, setting it to 128 would give you 50%, and setting it to 192 would give you 75%.