I don't see why a theory of counterfactuals couldn't apply to mathematical propositions. After all, our cognitive architectures use causality at a primitive level, and the same architecture is taught math.

And certainly, while learning math, you were taught results that didn't "seem" right at the time, so you worked backwards until you could understand why that result (like 2+6 = 8) makes sense.

So you just have to imagine yourself in such a similar situation about math, learning it for the first time. If everyone in class seemed to understand multiplication but you, and it were also a fact that 3*3 = 8, what process would you figure was actually going on when you multiply? Then, apply that to 13*3.

"What is 13*3, if 3*3 was 8?"

To this I ask: "Which 3*3?". The whole procedure is something that is done with a description of program (system), and any facts of which we can speak as holding for the system are properties of the system's "mind". Thus, the fact of what 3*3 is must be located somewhere specifically (more generally, as a property), for it to be meaningful to talk about this fact in relation to the system. You are considering interaction between this fact, as parameter, and the rest of the system, and this activity requires seeing both on equal rights.

When you, as a human, reading the question, you may try to interpret it as pointing to a specific subsystem, as I did in the post. More generally, the question is only meaningful in this way if it admits such interpretation.