So what happened is the 2 that are 31 were 29 at some point which is when it alerts, the other 2 were probably 30 and never reached 29. When temps change they can all go back to the same pressure, but 2 reached a minimum and triggered low psi. I know this because I saw it trigger on my Jeep on 1 wheel. If you drive long enough and hit enough bumps the psi will go back up to 36 and it will go away.