Why can’t SEO results be guaranteed?
Google sets their listing automatically. SEOs don't have access to the administration of the Google.fr website.
Using all they know about Google's algorithms, they try to optimize a website in order to end up with a better ranking. Unfortunately, we can't be sure that Google will place your website ahead of the others that may be working as hard as you are to get them ranked.
On complicated keyphrases made up of 5 or 6 keywords, a good SEO specialist can rank a website because few people worked on this kind of keyphrases.
But on all competitive keyphrases (ex "repo agreement"), it is way more risky to guarantee results. A guarantee in referencing will be considered as a risk taken by the specialist, like a commercial strategy but you will have to be aware that nobody will force Google into displaying your website in first positions of engines.
Be careful on the choice of your keywords. A keyphrase made up of several keywords in the plural will certainly bring you a turnover close to zero. As a consequence, being on the first position will only gratify your ego and be useless.
Google will always have the final say.
Furthermore, it is even more risky for a SEO professional to ensure results when they don't know what kind of work will be carried out at the same time by the competition. The SEO specialist can also be dependent on the quality of your processes.
A slow server or one that frequently crashes (not to mention security) can cancel or wipe out months of work.
In the same way, a wrongly scheduled redesign with non-redirected or destroyed URLs can wipe out a lot of work in a few days.