The designers of GPS system time did understand. GPS system time is *about* 19 seconds behind TAI, not exactly 19, and the difference varies by nanoseconds. Perhaps nanoseconds is unimportant to some, but for others nanoseconds is very important. That makes them not the same, so choosing a separate name and offset avoided any confusion that they were the same. Think of it like a trademark issue; there are many kinds of cola, but TAI is a name for the original cola, and only BIPM is authorized to make cola with the name TAI.