When it's sunny, the driver likes to turn the interior lights off because there is already bright light coming from the windows. It wouldn't make sense if they have interior lights on when it's bright as heck out, but some ops forget and leave the lights on. On most buses I've been on, especially during the summer, the interior lights are off. I've been in some cases where the lights are on, but I'm just assuming that they forgot to turn them off.
For the reading lights on the express buses, they are more dimmer than the usual interior lighting. Especially at night, where people just wanna rest, they prefer more dimmer lighting since they would spend longer times on the express buses than the local buses. I think it's also aesthetically pleasing to have just the reading lights on, and customers prefer it better for the same reasons. These are all my assumptions.