The Windows USB game controllers dialog (joy.cpl) uses the legacy DirectInput API. Xbox-compatible controllers have special behavior in DirectInput that causes the trigger axes to be combined to a single Z Axis input:
The combination of the left and right triggers in DirectInput is by design. Games have always assumed that DirectInput device axes are centered when there is no user interaction with the device. However, the Xbox controller was designed to register minimum value, not center, when the triggers are not being held. Older games would therefore assume user interaction.
DirectInput API has poor compatibility with modern gamepads. These days, applications should use XInput or Windows.Gaming.Input. When Xbox controller inputs are read through XInput or Windows.Gaming.Input, the left and right trigger inputs are not combined.