Although Mike Poole is our resident currency expert, I'll throw in my dollars worth. That "error" is quite common.
The Treasury seal and the serial numbers are printed seperate from the rest of the bill.
If you take 10 bills and put them side to side you will note that there will be a slight difference with everyone.

I think that the margins were within allowable limits or the bills would have been pulled.

What is a real error is that every now and then a bill is discovered where the serial numbers don't match.