Reach counts people, not screens. If one person sees your post three times, reach goes up by one while views go up by three. That makes reach the better measure of how far a post travelled, and views the better measure of how often it was shown.
Reach is estimated by each network, and each one counts it its own way, so compare reach within a network rather than between them. A sudden rise in reach with no rise in engagement often means the post was shown to people outside your usual audience; a fall usually follows fewer posts, later posts, or a format the network shows less.
