Really? The human brain has over two petabytes of long-term memory. Does your computer have a hard drive that size?
You seem to imply a difference between a conscious being deciding to use math and a computer receiving instructions and giving outputs. But, if consciousness is deterministic, is there really any difference between a conscious being receiving inputs (from its senses) and deciding to use math, and a machine receiving inputs (indirectly from conscious beings) and deciding to use math?
You have said that you believe humans are mostly deterministic. There are also many instances of people behaving deterministically. Among them are, as I've noted before, people who have transient global amnesia.