It's natural that the calculator application was among the first computer programs available. Likewise the calculator icon is among the first computer icons.
The sample that is featured here is from the Xerox Star. There was a calculator application in the Xerox Alto also, so it is likely that there was also an icon for it. The calculator as a device is gradually becoming obsolete. Therefore the classic calculator icon is likely to seem somewhat arbitrary to future generations. Already the design of the icon in modern devices is putting less emphasis on the physical device and concentrating more on the mathematical symbols.
Xerox Star 1981.