How did he get into the house? (spoilers)

Ok, I loved this movie and I'm not trying to be critical but one thing really bugged me.
During the confrontation between Sally and her husband, how did Luke get to the door?
Earlier in the movie they made a big point of showing that it was a big problem for Luke to get into the house without a ramp. Sally had to hold his ankles to help him in when he came to her place for dinner. Later on they built a ramp so the home would be accessible to him and then they eventually had to take it down because Bob was coming home.
But then during the confrontation scene, Luke just wheeled right up to the front door with no problem.
So if Luke could have just come in the front door on his own all the time, why did they go to all the trouble to go through the side door up to that point??
Hopefully, there's a logical explanation that I was too dense to see.
Can anyone help me out on this?