Why when people use my phone does it sync them to my level?

Why? Because a phone is supposed to be a private, personal device to which you do not give other people access. Much of the game record is kept on your device and when others use it it simply replaces your records with theirs or their records with yours. One of the costs of your apparent disregard for security is therefore the game entanglement you are now experiencing and about which nothing can be done.