Where did you get this info? I would think it would be the other way around! I take Adderall, never have taken any form of methylphenidate/dextro-methylphenidate before. But I know with Adderall the IRs and the XRs are generally prescribed around the same dosages for each like if a doctor scripts you 20mg Adderall IR than they will probably give you 20mg of Adderall XR if you preferred to switch and vise versa. If I had to guess I would think the reason behind the Focalin IR and Focalin XR having such different max doses, I would say because it is well known that a lot of people like to abuse the IRs because they can easily be injected, insufflated, and tampered with in general, while the XRs are pretty hard to tamper with, or at least get as good results from abusing them in other methods than orally, from what I have heard, but again only have tried Adderall as far as prescription stims go. I am sure someone on here will probably have a better answer, this is just my thoughts, and things I have heard over the years.
To your second question a doctor can prescribe you what ever they want, there is no standard "maximum" dose that they cant exceed as far as I know, it all depends on how bad you need this medication, what dose works well for you, and of course the doctor! Some doctors hand stimulant medication out like candy, I have known people over the years on like 40mg Adderall XR along with 40mg of Adderall IR for breakthrough relief throughout the day for their ADD/ADHD. Other doctors like mine wont go over 20mg of either IR or XR Adderall. Also my doctor doesn't like to prescribe any methylphenidate based meds for some reason, they prefer to prescribe amphetamine based meds, like Adderall, Dexedrine, and Vyvanse if prescribing stimulants and I think because methylphenidate can be injected with much better results than Adderall, and I get them from a suboxone doctor, so they have seen it all and know the abuse potential of all these meds. So yeah, every doctor is going to be different on that aspect of prescribing more than the suggested dose , less of it, or the actual suggested dose of either IR or XR stims.