Settlers mappers

Something have to be wrong with mapping. I sent them to maps with 10-20% chance for death and 10 times in a row they die on first or second map with 5% completion. It's just 7 and 8 tier maps and i keep on losing them. Is this some bug where it shows wrong % for death?

10%-20% per mapper, so with 6 mappers that's 47%-74% chance of death.

Don't run if it's higher than 1%.