Dogs Bark When Visitor Re-Enters The Room

Both my dogs bark when the doorbell rings or strange guests enter the house. I'm planning to start working on desensitizing the doorbell, but the first time strangers walk into the house doesn't really bother me. It's more the barking at visitors they already decided were ok and no longer needed to be barked at when they sometimes walk in the room. I can't seem to figure out why and it seems random to me. For instance, my parents are visiting this week and they have been here for 4 days so far and the first morning and this morning, they barked at my parents when my parents first walked into the family room after waking up. Then they almost always bark if my parents ran to the store but sometimes they will bark if my mom runs upstairs to grab something from the guest room (or my dad comes out of the bathroom, etc.) but not every time. My parents aren't really dog people so it kind of makes them feel unloved by the dogs. My parents have been trying so hard, my mom plays with them and my dad gives long scratch sessions, which my dogs love. What is going on and what can I do to resolve this?

I should mention that one dog is shy about strangers by usually warms up within a half hour while the other one generally loves people, including new people.