Thing is, if you follow the recipe AS EXACTLY AS POSSIBLE twice, you are likely to get TWO DIFFERENT RESULTS.
A patent is meant to protect your tricks for following the recipe, such as you guarantee the same result both times. The recipe tells you what to do, the patent describes how you follow the recipe - how *you* *personally* follow the instruction that says "cream the sugar and butter together".
Thing with Maths, there's only one way to follow the instruction, therefore you always get the same result, and anybody else following the same recipe (algorithm) will get the same result.
Even something as simple as "bake on a low heat for 10 minutes" will have twenty people doing it forty different ways!