SingKey scores all twelve keys against your range: how much of the song sits inside your comfortable range, how well it lines up with your best zone, how far it pushes the edges of your voice, and how far the band has to move from the original key.
The number next to a key is a fit score, not a confidence. SingKey does not yet have the evidence to tell you how confident it is — that needs comparing its recommendations against what real singers actually choose, and that work has not been done. It will not show you a confidence figure it cannot stand behind.