It originally referred to a particular calculator glitch, when using octal mode, which is base 8. Only digits 0 through 7 are supposed to work in octal.

But there was at least one model calculator out there that would still allow you to type in the digits 8 and 9 in octal mode, though those digits should have been ignored and disabled in that mode.

9 in base 10 converts to 11 in base 8, which the calculator automatically does in the background before performing the calculation, so the calculator actually ends up calculating 11 + 10 = 21, which in base 8 is actually correct.

Now how that got misconstrued into memes such as this one, I’ll never know. 🤷‍♂️