I wouldn't necessarily chalk it up to intelligence. I'm finishing a masters in statistics at the moment, quite a lot of math involved, and I'm mostly doing fine

I wouldn't necessarily chalk it up to intelligence. I'm finishing a masters in statistics at the moment, quite a lot of math involved, and I'm mostly doing fine, however when I started the program after not having seen any mathematics in about a decade, I hardly remembered the difference between the circumference and the area of a circle. Now, calculating the amount of paint needed to paint a room might seem like more of a life skill than a math skill, but for some people it just doesn't come up, and when they do suddenly need math for something, they're not likely to trust whatever basic arithmetic they do think they still know. For some people who are weak at math, it even becomes a badge of honor: "yeah, um, sure, we could calculate how we want to split the bill, but do we really want to be such nerds about it?" Studies abound, too, showing that even grade school maths teachers often eff up when quizzed on pre-algebra.