> A double-precision real in the 8087 provided ~15 digits of accuracy, because that's the way Berkeley floating-point expert William Kahan designed its number representation
Kahan was the co-architect of the 8087. Palmer (at Intel) hired Kahan as a consultant for the 8087 since Kahan was an expert on floating point. Kahan says: "Intel had decided they wanted really good arithmetic. I suggested that DEC VAX's floating-point be copied because it was very good for its time. But Intel wanted the `best' arithmetic. Palmer told me they expected to sell vast numbers of these co-processors, so `best' meant `best for a market much broader than anyone else contemplated' at that time. He and I put together feasible specifications for that `best' arithmetic." Kahan and Palmer were co-authors on various papers about the 8087 and then Kahan and others authored the IEEE 754 standard.
And Kahan's Turing award: "During a long and productive relationship with Intel he specified the design for its floating-point arithmetic on several chips starting with the 8087" https://amturing.acm.org/award_winners/kahan_1023746.cfm
I don’t think Kahan had a direct part in the design of the 8087. https://en.wikipedia.org/wiki/Intel_8087#Design_and_developm... agrees, saying “Palmer credited William Kahan's writings on floating point as a significant influence on their design.”