Why would i have the same employee id number after being fired, and then brought back as a new employee?

I was fired from my job back in February, after moving to a new position. I saw a couple of weeks ago that they had an ad out for my original position, and called to inquire about it. At the time I was told that that ad was a mistake, then received a call two days later, to talk about potentially going back to work with them. Since the timing was good, and I was denied unemployment, I agreed, to the terms of looking 1.35 an.hour, and starting over as a new employee. When I got my first check, I noticed I had an original hire date of when I came back from a layoff last year, and my same employee id number. I figured if I was supposed to be starting over, complete loss of seniority, and all, as a new employee, why would I have a id number that goes back 5 years. Would I be right to assume I still have my seinority? I was asking my supervisor about it, but he didn't really have an answer.