Bed bugs didn't leave after exterminators came, why?

The exterminators came last Thursday and exterminated my entire apartment. Up until today, Wednesday the 22nd, I haven't had one bed bug bite, nor have I seen one. Today, I just woke up due to a pinch on the left side of my waist. It was a huge bed bug, which I squished. What I'm asking is WHY? Why do I still have bed bugs when the exterminators came. They did say they were going to come a second time just in case, but really, I thought it would be done after one job. What's the probability that they will be gone after the second extermination? Is it possible for them not to leave after the second extermination? I can't just keep calling these guys for the rest of my life.