Why does my mom not hear or understand me, but seems to understand others just fine?

My mom is 81 years old ahs demetia and now lives with me. whenever I try to tell her something or answer what she may ask, she either says I dont understand you,or I cant hear you. But she can almost hold a conversation with everyone else without sayin she cant hear or dont understand. I have steps in my home leading to the bedrooms,she crawls up on hand and knees like a baby every night and comes down in am on her but like she is 2 years old. I understood this because her home had no stairs. But now the physical theraphist came today to evaluate her to see what services they could offer her and I, she walked up the stairs straight like she was 71 instead 0f 81. She was nervous but she rushed up there like she was trying to prove a point! He asked her to show him how she gets in and out of bed, and she laid down on her back and started doing bicycles twirls like she was excersing! But at night she's rubbing ointment on her knees!!! Can you fake demetia? (just a little humor)