That is right turn off breakers until you are sure the power is off. Most likely if someone switched part of the receptacle it is wired to the same circuit as the plugs.
I do know a lot of times a contractor will put lighting on one phase and outlets on the other to balance the load, but again I don't think there is NEC stating it must be done this way.
This would not be good practice, or even be a balanced system. If you want your circuit to have balance, it would be necessary to wire the same amount of lighting on each phase, and likewise for receptacles. By doing this a balanced home would have virtually no current on the neutral. Or just the unbalanced part.
Putting lights and plugs on different circuits has different advantages.
1. The power to plugs can have more current available for intermittent loads etc.
2. The lights don't go out when you trip a breaker from overload
3. Lights can be wired with 15 amp circuits and plugs on 20 amp
allowing the 3way and spacial switching circuits to utilize the less expensive #14 gauge wire.
4 Control and other features included, may have personal reasons.
The plug in your house may have two different circuits to it. The reason might be very specific but it is most likely
because the person who owned the house did it and he was very pickey for some reason. There is no real good reason to control a convenience light in that manner, that I know of.
