"not possible" is quite a strong claim in the realm of concurrent software, and so is "simply". Benchmarking is taking a measurements of a nondeterministic process, i.e. a statistical sample. Yes, some problem with the setup is quite likely given the result, but other explanations are possible as well. What if some threshhold for some action that helps the server deal with more concurrent connections was just not hit at 100 connections? unlikely but not imposible.