If I understand correctly AVR datsheet, partiularly for ATmega328p, section 15.9.2, OCRnA value is calculated by:
OCRnA = F_OCnA/ F_clkIO / N - 1
For instance, for 1s timer, prescaler 1024 and 16-bit timer, the value should be 15624. On the web page result is 15625.
On various sources, calculators shows either result, but according to datasheet, only one is correct.
Perhaps also to add prescaler value and resolution for complete calculation.