I too had heard the legend that Excel had been a Mac product, but after some research I have come to believe that was based on VisiCalc's port to the PC that was the genesis of that legend.
It’s not just a legend that Excel was first published for the Mac, it actually happened. For some details see:
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History of Microsoft Excel 1978 – 2013 -
A Brief History of Spreadsheets-
SpreadsheetMicrosoft developed Excel to succeed Multiplan (its answer to VisiCalc, and a spreadsheet which was in turn eclipsed by Lotus 1-2-3),
with the intention to, in the words of Doug Klunder, 'do everything 1-2-3 does and do it better’.
