It's odd because I've never seen an RPi header on anything but an RPi, but Arduino is ubiquitous. I find it hard to believe some scrappy maker enthusiasts in 2005 accidentally changed every vendor's form factor, especially considering they can't even agree on an IDE, and those came first. Seems like there must have been an consortium or a spec or an RFC that was agreed upon? Odd that arduino.cc doesn't mention it.