There are 3 possible conditions that the undercarriage can be in, that the pilot needs to know about.
1. Gear safely DOWN ie locked & in a useable condition / all doors where they should be etc
2. Gear safely UP ie correctly stowed & all doors where they should be, allowing flight at speeds outside the gear DOWN limits.
3. Gear not in the safe UP or safe DOWN condition.
Allowing for always having a green light to to indicate the most important condition ie DOWN, then the remaining 2 states can be indicated with just one more light.
The two options are:
1. Use a light on to indicate safely UP, off to indicate not safely UP
2. Use a light off to indicate safely up, on to indicate not safely UP.
Cessna have chosen option 1. Piper tends to choose option 2.