Think of it like top down budget va bottom up budget.

RAMS engineers do a bottom up analysis. However, it is fairly typical for the to be subtlety biased in a way that changes the final answer. (Usually by ‘rounding up’ on individual hazards).

Top down takes an outside view and says ‘this looks like these other past projects. It should land in around the same place.’

I’ve found top down tends to be more accurate project wise. Bottom up more accurate for specific subsystems.