[sheetjs]

Excel predates RFC4180 by nearly 20 years (RFC4180 is October 2005, Excel 1.0 was September 1985) and this behavior was already cemented when the RFC was written.

As for the actual RFC, it's worth taking a read. Any sort of value interpretation is left up to the implementation, to the extent that Excel's behavior in interpreting formulae is 100% in compliance with the spec.

[dtech]

CSV predates Excel, and other CSV implementations don't have this behavior

[throwaway_pdp09]

What spec?

Anyway the RFC doesn't mandate any value interpretation IIRC.