I've seen this limitation brought up before, but I don't have an intuitive understanding of when it would apply. Is the essential issue spacing between floats?

The scenario I'm thinking in is each user's entry is a real number from U[1e15,1e15+1], and we want to output sum. The sensitivity is still 1. So the Laplace Mechanism outputs something like sum(DB)+laplace(0,1/eps), which will be exactly sum(DB) because laplace(0,1/eps) is smaller than the gap between floats >=1e15.

For this to be an issue, the actual sum(DB) needs to be implemented in full precision right? Otherwise it seems like the sensitivity isn't necessarily 1.