1-in-270 is overall probability threshold for a 210 day notional ISS stay.

For the journey home from ISS to Earth, the probability threshold is 1-in-1000. Likewise, it is 1-in-1000 for the journey from Earth to ISS.

The riskiest part, which increases the probability from 1-in-500 to 1-in-270, is the ISS stay – the extended stay in space is faced with a continuous risk of micrometeoroid damage.

Its kind of grim to think a trip to the ISS has a 1:270 chance of death just from the unavoidable roulette of getting zinged by a micrometeroid.

1/270 is the total risk from all causes. Just the risk from micrometeoroids would be (at most) 1/270 - 1/500, which is roughly 1/587 (0.17%).

Welcome to space, it's really fucking inhospitable and will always be.