The thing is that different job functions that are exposed to overlapping user bases in the same industry (and even office) use a different 20%. If it was just per-industry variation, it would be easy for industry-specific competitors to succeed, but the parts of Excel used actively by Andrew, who makes a tool that is also consumed by Bob and Carol are different than those used actively by Bob and Carol (which also differ from each other, and which each have their own users who rely on the tool and the features it relies on, even though they don't actively use the features.)