Preferences not working

Every time I step away from my iMac for about 5 minutes it asks for my log in password. Below are my preference settings. I tried trashing the associated preference files and nothing changed. Any ideas on how to troubleshoot this issue?

Thanks.

iMac Mid2007

OSX 10.11.6

P.S. I don't understand the security logic when I sign in to Apple with my Apple password and they send a 6 digit verication password to the same device that I'm signing in from. Besides the fact that I've signed in from this device hundreds of times and the message says "you are trying to sign in from a new device please enter the verification code we sent". Isn't the number supposed to be sent to another device, thereby creating some security.