First of all - this is why some people prefer a smaller library system.
Second - because it's such a large system, I have no idea how they process holds, and how they arrange deliveries. Does the branch nearest to your branch with a copy send the hold over? Or is it a random choice? How quickly do books get from one branch to another? Even in our small system of three locations, there are certain places that won't get their stuff on the same day, due to delivery routes. I'm sure this is true for OCPL.
Benefit of the doubt - you placed your hold, and one minute later someone returned the book at the farthest branch (which, for OC, can be pretty far). As it was the very next copy that saw a scanner, that exact copy was stuck on hold for you and is making it's way down the chain of deliveries to you.
I'd recommend that you call your branch and ask what the status is on your book. They should be able to say where it's at, and estimate arrival time. Don't forget to get very mad over this and yell at them for purposely screwing over your reading groove.