We have been researching, studying and deploying good SEO tactics since 1998! SEO in large has changed a lot since 1998, but the basics have remained intact.
Over the years, we have developed a number of systems for our SEO work, each tailored to a VERY specific kind of SEO. While in the old days, SEO used to be pretty straight forward and singular, it just no longer is. EVERYTHING is based on an algorithm, and there are a whole lot of them!
Websites in different industries require distinct kinds of SEO. Tech is content heavy; Ecom is image heavy; Real-estate is page heavy; but you can rank a local photography business with one page and almost no text! Every Niche is different than the other, and the SEO has to be as well.