The are many things which are difficult to measure directly, and thus people tend to use indirect measurements instead; school-learned math is full of such things. Consider a fairly simple example: "What is the factorial of a number?"
Determining whether a student knows this is going to take a bit of work (particularly if they lack a formal way to specify it e.g. Pi notation or even a programming language), but we might approximate it by instead asking "What is the factorial of 5?" Now obviously this is not a perfect measure of what we're actually looking for even in the absence of calculators (e.g. someone might memorize 5! = 120), but it's easy to evaluate and is probably a decent proxy in the absence of calculators.
Determining whether a student knows this is going to take a bit of work (particularly if they lack a formal way to specify it e.g. Pi notation or even a programming language), but we might approximate it by instead asking "What is the factorial of 5?" Now obviously this is not a perfect measure of what we're actually looking for even in the absence of calculators (e.g. someone might memorize 5! = 120), but it's easy to evaluate and is probably a decent proxy in the absence of calculators.