Algorithms aren't patentable. What's patented here is a specific circuit that implements this algorithm in order to calculate the average in a single instruction cycle.
this would require a lawsuit to find out, but this wording in the description suggests the authors’ intent with the claims was to patent this method of computing an average on any processor:
> A general purpose computer or processor with suitable circuitry can execute the invention in a single instruction cycle (as is preferred) or multiple instruction cycles.
LZW compression[1] was patented, and Unisys actually had success extracting license fees with it[2]. In that sense, clearly an algorithm was "patented enough" that the patent was granted and used, even though the patent is literally just math. The MP3 patent is also just a mathematical algorithm which had even more legal success.
So, while technically algorithms are not patentable, in reality, the USPTO will grant patents for algorithms if you write enough legal gunk around them, and the difference doesn't really matter when a patent troll is sending threatening emails and your lawyers are demanding 20x the licensing fee to take the case.
[0]: https://en.wikipedia.org/wiki/Alice_Corp._v._CLS_Bank_Intern...
However, this is also why business method algorithms like the Amazon "One-Click" are not patentable in most countries. There is no trivial theoretical equivalence between one-click shopping and an electronic circuit.
In the US, unfortunately,they can be (although not all algorithms are patentable). For example algorithms used in many media formats are patented.
Edit: but back in the 70s the USPTO didn't have the search databases they had in the 90s or 2000s. It's more the XOR patent being wielded in litigation that was extra controversial