The similarity assertion ensures that the expected and actual states are structurally similar. It uses a score based on the number of elements, their positions on the screen, and their attributes. It does not take into account how they look pixel-wise.
From now on, you can always inspect screen structures by hovering the similarity assertion (both in runs and tests modals). Comparing the actual and expected screens that way can usually explain why they look pixel-wise identical but have less than 100% similarity.