util/delay.h from arduino _delay_ms not delaying in accurate time with my hardware configuration
At a 30000 ft. level, it's because the defined F_CPU value doesn't match your hardware. This can happen because the incorrect device is chosen in the IDE or because the device is misconfigured in boards.txt, or because the hardware is somehow different from what is expected.

Given that you are using a bare ATmega328P, assuming that you've selected the correct crystal, and that the apparent time scale is 8x, it is likely that the CKDIV8 fuse is set. This means that the chip is running at 1/8 the crystal speed. You will need to unprogram (set to 1) bit 7 of the low fuse byte in order to reset it. Use AVRdude from the command line to read the low fuse byte (-U lfuse:r:-:h), set the bit (lfuse | 0x80), and then write it back (-U lfuse:w:0xnn:m).