Wilsons do give out scores but only to those who have failed the test. Boys who have passed Wilson's test do not get told their score at any stage.
Sutton do not give out scores and go one further in explaining why:
Boys who pass the eligibility test, and who choose to take the second stage test will not be given any further information until after National Offer Day (1st March). The reason for this is that knowledge of rankings or scores could allow parents to work out the school offer they will receive, so could be interpreted as an offer of a school place. Schools are not permitted to release any information which could be interpreted in this way.
....which begs the question about where that leaves Tiffins and Nonsuch (not to mention countless grammars around the country that give out scores plus a traffic light system to show how likely a place is!).
The upshot however is as you say: Tiffins and Nonsuch give out scores that can lead to guesswork. Wilsons and SGS do not give scores to boys who pass their tests. Nobody who has passed the SGS or Wilsons test knows whether they are likely to be offered a place.