If you completed a beatmap, you will be presented with the results screen, and hovering on the Performance Graph will show you this:
This got me thinking, osu!catch is not really a timing based game, so why does it show the unstable rate and hit error? Where does it get these values?
According to the osu!wiki:
Unstable rate represents the consistency of the timing of the hits, where lower numbers are better (top players often score below 100). Note that the value measures consistency, not accuracy, [ ... ] The formula is essentially the standard deviation of the hit errors (in milliseconds) multiplied by 10.
Here's the problem: osu!catch is not a game mode where you get high accuracy by timing the notes but you get high accuracy by catching more notes (or fruits). The osu!catch's accuracy formula shows this.