Robin is bulimic.Look at the episode where Robin and Dojimma go undercover as waitresses at Harry's and have to wear fanservicey uniforms. Don't Robin's bones look like they're protruding? (She is seen eating food in a few episodes, which rules out anorexia.)
Robin doesn't need her glasses.Sure, she's told that the reason her abilities go haywire is short-term myopia; but Robin is rarely shown looking through her glasses when using her powers. It could be that someone in the STN-J just realized that Robin needed something external to boost her self-confidence. Perhaps Amon?
The series takes place in the same world as Painkiller Jane."Witches" and "Neuros" are different names for the same thing; people with Superpowerful Genetics; many of whom are abusing these powers and bringing down the attention of extra-legal agencies who hunt them. It is commented in WHR that America handles Witches "differently."
The series takes place in the same world as X-Men.For many of the same reasons as above. "Witches" and "Mutants" seem to have an awful lot in common, and they've both got government agencies plotting their downfall in less-than-ethical ways. In fact, with only a little analysis, you realize that since this is set in Japan, (given that we already see a few of Robin's difficulties grasping certain aspects of Japanese culture), so it's safe to say that it is just a cultural difference that leads to the group called "mutants" in the US being called "witches" in Japan. Different countries have different reactions to mutants and different names for them. Also, despite the X-Men supposedly having international impacts and effects, much of what we see about the is produced in the US, and Americans often over-estimated how much American news spreads to the rest of the world, and how much activities in America affect everywhere outside the US.
The series takes place in the future of one of the alternate universes Homura created to save Madoka.In one of the worlds, Walpurgisnacht was defeated, though Madoka still either died or became a witch and was killed. While Homura left for the next world, everyone else in the world got on with their lives. Eventually, some puella magi settled down and had children with civilians. The result was a new breed of "witch" in the form of the descendants of the undead puella magi. This one remains human but is still prone to insanity caused by despair. "Orbo" is a liquid form of what pureblood Magical Girls would call a Soul Gem. Kyubey collects a great deal of energy when girls 'transform' from "Craft-User" to "Witch".