I have a question about what seems to be modular arithmetic, but I can't quite get the answer.

The problem goes along the lines of:
It is often said Earthlings use the decimal system because they have ten fingers. We see a Martian write down the equation:

$$
x^2 - 19x + 76 = 0
$$

When asked to write down the difference between the larger and smaller root, the Martian writes 9. How many fingers do Martians have?
Note: Martians write numbers between 0 and 9 exactly as we do.