(Pretty old thread but)
It's just a glitch. I had three springs on a map too, so I just used two to help the mice. As mentioned earlier in the comments, sometimes I get unlimited uses of the Chief's Food skill too.
When I noticed I had three springs for the first time, I asked the mice in my room why. They only said that some maps allow three springs, but that's not true.

This has been a thing for a long while now. This doesn't actually harm anything, since it only lasts for one round so I don't think this is a high priority to fix when there are other, more important things