The way I understand it is that time itself is altered by gravity and/or velocity. So atomic decay that occurs on a very specific cadence in each reference frame will not occur simultaneously in 2 or more different reference frames that are not in the same gravity, moving at the same velocity. There’s even a measurable though very small difference in the passage of time between sea level and high mountains due to the difference in gravity. I’m leaving a lot out and there’s a bunch of math involved, but i think that’s mostly correct.