Are Sprint ESNs Accurate?

The last time I checked an ESN, they told me that the ESN was clear. However, when I tried to activate it, they told me that the ESN was bad. Why is that? I asked a friend and they told me that when Sprint (NYSE: S) customer service and reps do an ESN check, they don't really go through with a "trial activation" where they try to activate it to see if it's clear, instead they act on a list that isn't as up to date.