Blink period shorter than expected

I made ​​a very simple circuit for LED to blink.

The code is as follows:

# include <avr/io.h>
# define F_CPU 20000000UL
# include <util/delay.h>

int main (void) {
DDRB | = _BV (DDB0);
while (1) {
PORTB ^ = _BV (PB0);
_delay_ms (10000);
}
}

The processor is an ATMEGA324PA with clock = 20 MHz.
The fuses are:

lfuse = 0x4F
hfuse = 0x99
eFUSE = 0xFF.

The problem is that the period of switching on / off of the LED is 
approximately 16 seconds. instead of 20 seconds expected.

Where is the error? What should I check for a period of 20 seconds?

Thank you to those who can help me.

Domenico

PS: I suppose that with a quartz 16 MHz operation would be correct, but 
there is a way to use one 20 Mhz?



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