With the wheelchair thing, I would say "I don't know why...some people get hurt, some people are born with legs that don't work like ours, some people get sick...there are lots of reasons why people have wheelchairs". Even though sometimes you can tell it's a severe disability....it's easier to keep it simple so that the next time they see someone in a wheelchair, they know there could be any number of reasons why they are in a chair.
I come from a very conservative, religious area. A lot of people use the "that's how God made them". We don't go to church, but for some reason, my kids picked up on this early on (they went to pre-K at a church). My son had some developmental delays, etc, when he was young and I overheard someone ask him why he couldn't do something and he said "that's just how Jesus made me". I thought it was really cute.
Ironically, that was one of my son's issues....he had NO filter. He was about 5 when he went up to a person with dwarfism and tried to play with him. When the man gave him a weird look and kept walking, my son said "mom, he looked kind of weird". I think he heard.
I always had to prepare myself when I saw a person who looked different coming our way...whether it was a disability or obesity or piercings, etc. After a particularly embarrassing incident at a fast food restaurant, the very next day he started asking questions about a man in a wheelchair....RIGHT after we just had the whole conversation about "it's OK to be curious but you shouldn't stare, and you can ask me questions, but not until later when the people are gone."
I remember one time when he was 3 he and he was just starting to talk he said "wooook!!" (look). He was pointing at a deformed boy at his school. "Wook shoes!". He had big styrofoam balls for shoes due to his deformity. I remember saying "yes, I see those big fancy shoes!" I was also pretty happy that my son was putting words together, as he had speech delays. The boy looked upset by my reaction. I guess he was used to people just hushing their children and making them look away.