Glen, the gain on the camera follows a logarithmic scale, so going from gain 139 to 100 is 3.9dB gain ~1.57x ... so a comparable exposure is 1.57x 60s. Exposing for 120s is collecting more signal, which should explain why it looks brighter. Nice shots for short exposures
Obviously, like Ray says, you only really need to exposure your desired feature over the read noise. Pretty exciting stuff with this camera having such low read noise.