I can't say that I am completely well-versed on just how these things were ascertained, but it's a quote from the RIC, etc. We're speaking about the time of Hadrian -the mint at Lugdunum, which had been for nearly a century the main mint for silver and gold while AE's continued to be struck nearly exclusively at Rome, was no longer striking the majority of AR and AV for the Empire at some time during the reign of Nero.
This re-centralization ocurred during the time of the Nero's debasement of the denarius. After this time, evidently, Lugdunum's mint was either closed or used only sporadically, mostly for AE issues, through the Flavian Dynasty. By the time of the Adoptives, Lugdunum seems no longer to be in operation and all Imperial issues seem to originate in Rome.