top artists and songs lifetime change??

i checked my stats and the first few top artists are correct, but suddenly a lot of artists i am sure i havent listened to much have gone up and many just appeared. for my songs its a similar thing. a song that was in my top 5 isnt even top 50 anymore, and many songs poppef up that were my top in 6 months and a lot of lifetime songs went down. im confuseddddd.