The purpose of algo/SOP is to get identical results independent of the person executing the algo. The Euclid algo will produce the same result no matter who executes it.
Airlines have SOP for various contingencies (e.g, radio malfunction). Any pilot qualified on that aircraft executing the SOP will achieve the desired result (landing the plane safely). This is an example where SOP equals an algo.
Your tax example demonstrates the opposite. Five accountants can come with five different tax payments -- using the identical input data. Even basic definitions such as marital status or address (let alone terms like income and expense) can be disputed. Having a set of rules is not an algo.