Though your overall point is fair, the difference between 4.36 and 4.39 isn't overly significant, IMO. But the difference between 4.36 and Branch's 4.58 is pretty stark.
To also answer Nate's question, I think 40 time is relevant as part of the overall concept of RAS, which the Packers seem to value quite high. You have a guy like Branch that has been linked to Green Bay in mocks, but some note that he may be best to rotate to slot in nickel coverage. At 4.58, you worry about him getting torched by speedier slot guys in that role.
Also, the overall point is that there are a lot of bodies at safety, but mostly jumbled in the 2/3/4th round range. No one seemed to elevate themselves above the rest of that grouping at the combine. Essentially, these guys already had something pushing them down a bit, and then compounded it by not really running that fast.