julia> 3.321155762205247e-10 * 6.022e23 * 0.5e-3 * 1e-6
100000.0
julia> 3.321155762205247e-10 * ( 6.022e23 * 0.5e-3 * 1e-6 )
99999.99999999999
It’s about floating point numbers and how they are represented in binary machines.
Last time we had this was here: Mismatch between nextfloat(0.0) and eps(Float64)