That's not the time it was compiled, it was the time it was touched. When Apple builds the IPSW, they go through the firmware's directories and touch each file with a set time/date or the current time/date. They then encrypt the disk images, then touch those and the other files within the IPSW. It was compiled from the source code over many weeks as it was put together in chunks, not the whole thing at once. Thus, the touching to make it uniform.