My GF's mother has Rheumatoid Arthritis. She can walk normally for short distances and has to rest. The pain is severe, and not localized, so she has no noticeable limp. When she walks, EVERYTHING hurts.
My mother has emphysema. For quite a while, she wasn't quite bad enough for an oxygen tank, but still could not walk far without having to take a breather. Now she is on oxygen 24/7.
Just because someone appears to be walking normally, does not mean they don't have a problem requiring a close parking space.