The way the Google search engine works is based on computer programs, complex algorithms developed by engineers/researchers.
Good SEOs know how the main algorithm works regarding the elaboration of its ranking.
Google also communicates on the outlines to be respected in order to be well ranked in this search engine.
However, the knowledge and respect of all these criteria are not enough. Even a Google’s engineer would not be able to rank systematically on rank one.
Indeed, it is difficult, not to say impossible, to act on some criteria, especially those listed in the ‘off-page’ category.
A good SEO professional will therefore not guarantee you these ranks. Obviously, it is possible to estimate a 99% chance of ranking on some very uncompetitive key phrases.
If a provider guarantees you good positions in the natural SERP of search engines, be careful !