old mixed with new

I have four places that I get my notes from and in the last month I have noticed that I am getting the old style of motes mixed in with the new style. Now it was my understanding that the old style when they hit the banks would be pulled from circulation. So what is happening I could understand one maybe two slipping through but in the last 5000.00 purchase I had 47 old style bills. What's up with that?

I LOVE old style notes (in proper condition).
I like that the odd one still surfaces (not too often here, tho)

picked up another "old" 10 and a 1975 series $100 today from a friend who knows that I like to collect old notes.