Why this was allowed to be patented? This is a pretty much obvious thing to do when you have this kind of limitations.
It seems like people reviewing patents don't really understand what they are about. There is plenty of stuff like this in audio space.
E.g. this one https://patents.google.com/patent/US20170060527A1/en
"If you have a lot of patents and somebody comes after you, chances are one of your patents will overlap enough with one of their patents that you can negotiate a deal so nobody gets hurt. Whereas if you don't have anything to offer and you have nothing in your stable of patents, then you're stuck"
The article I sourced this quote from can be found here: http://summit.sfu.ca/item/7720
I just chalked this kind of weird specificity up to the legal minutiae required to patent musical hardware. Maybe someone with legal experience can weigh in on this. I'm guessing that if you go into this level of detail regarding the technical implementation, it probably makes it much easier to litigate patent infringement later, and narrows the possibility that someone else can get away with a similar implementation. The titles of some of Yamaha's other musical patents are pretty obscure: "Electronic musical instrument with user programmable tone generator modules (5,040,448)", "Electronic musical instrument capable of varying a tone synthesis operation algorithm (4,558,857)", etc.