Different results within same macOS GPU family

As I pointed out in another thread, I'm trying to port my OpenGL app to Metal, but it seems that every GPU has different results. In particular, I tried running the app on a MacBook Pro 2012 with integrated Intel Iris 1536 MB graphics, and received a report from a user running it on a MacBook Pro 2014 with integrated Intel Iris Pro 1536 MB graphics. According to this document, the two Macs should belong to the same macOS GPU family 1 (they are both running macOS 10.14.6). Now, is it really possible that they are producing different results?