I don't agree those are casting bubbles. If you do an ACSEARCH hunt on Julius Caesar, for whatever reason, many of them are struck significantly off center. What you will find is that the part of the planchet that is not struck often has some odd bubbles outside the die strike area. Really. Go do the search.
What always fascinated me is that what does this say about the planchet manufacture? Did the Romans use some sort "master magic tiny ladle" for each planchet? Did they use some sort of "drop shot" with molten silver. I haven't dug far enough into sources to understand planchet manufacture to even know what knowledge is out there.
Paul