Electronic – avr attiny84: wrong delay

I'm pretty new to avr programming. I'm facing a strange problem that I can't solve so far.

I've wrote a simple code:

#include <avr/io.h>
#include <util/delay.h>

int main(void) {

    DDRA = 0XFF;

    for (;;){
        PORTA = 0xFF;
        _delay_ms(1000);
        PORTA = 0x00;
        _delay_ms(1000);
    }

    return 0x00;
}

I'm setting the F_CPU (the value used by _delay_ms() ) trough the Makefile I'm using to compile and upload the code:

DEVICE     = attiny84
CLOCK      = 20000000
PROGRAMMER = -c usbasp -P /dev/tty.usb* -b 19200 
OBJECTS    = main.o dallas_one_wire.o
FUSES      = -U lfuse:w:0x62:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m


######################################################################
######################################################################

# Tune the lines below only if you know what you are doing:

AVRDUDE = avrdude $(PROGRAMMER) -p $(DEVICE)
COMPILE = avr-g++ -Wall -Os -DF_CPU=$(CLOCK) -mmcu=$(DEVICE)

# symbolic targets:
all:    main.hex

.c.o:
    $(COMPILE) -c $< -o $@

.S.o:
    $(COMPILE) -x assembler-with-cpp -c $< -o $@
# "-x assembler-with-cpp" should not be necessary since this is the default
# file type for the .S (with capital S) extension. However, upper case
# characters are not always preserved on Windows. To ensure WinAVR
# compatibility define the file type manually.

.c.s:
    $(COMPILE) -S $< -o $@

flash:  all
    $(AVRDUDE) -U flash:w:main.hex:i

fuse:
    $(AVRDUDE) $(FUSES)

install: flash fuse

# if you use a bootloader, change the command below appropriately:
load: all
    bootloadHID main.hex

clean:
    rm -f main.hex main.elf $(OBJECTS)

# file targets:
main.elf: $(OBJECTS)
    $(COMPILE) -o main.elf $(OBJECTS)

main.hex: main.elf
    rm -f main.hex
    avr-objcopy -j .text -j .data -O ihex main.elf main.hex
# If you have an EEPROM section, you must also create a hex file for the
# EEPROM and add it to the "flash" target.

# Targets for code debugging and analysis:
disasm: main.elf
    avr-objdump -d main.elf

cpp:
    $(COMPILE) -E main.c

Regarding the attiny84's data sheet, it should run at 20Mhz under 5 volts.

Unfortunately the led isn't blinking at a rate of 1 every second but somewhat really longer around a 10 secs rate.

By tuning the F_CPU value, I've reached the 1 second blinking rate by using F_CPU = 1000000 (1Mhz)

Does that means that the attiny84 is running a 1Mhz or am I wrong somewhere else ?