"Breathable" is only half of the atmospheric status. For example, if it is Temperate/Breathable, no atmospheric damage should be taken afaik, but if it's hot/breathable you need Thermal Survival, cold/breathable needs Arctic, etc etc.
Why is it that out of the dozens of people who have spent extended periods on this cold/breathable planet, only one of them took damage? Glaciers aren't listed as restricted for Corellians. All of us have been walking around in armor with no ill effects.
My psychiatrist told me I was crazy and I said I want a second opinion. He said okay, you're ugly too.
Restricted terrain has nothing to do with damage - that just determined whether you can walk on that terrain without any items (like needing an aquatic survival suit to move on ocean terrains if you are not Mon Calamari etc).
Since apparently atmosphere damage has been changed to once a day or something rather than once an hour, its entirely possible the other guys got inside before the damage timer activated. Or they really were wearing the correct survival suit.