What the heck? I'm so confused. MIL called and sounded perfectly normal.

For the past couple of years MIL has been showing distinct symptoms of dementia. She sees people, thinks everyone is stealing from her, thinks husband is dating Walmart employees, accused the mail lady of being a hore and stealing her mail - then sneaking in the house and leaving it all on the bed. The list goes on and on. Now we get a phone message where she sounds 100 % normal. How is this possible.