I bet that is because of lengthy questioning to these guys before the trial. Often they are excessive to the point where they said "ALRIGHT yeah I am guilty so you can fucking shut up and I go sleep" Has that happened? Oh yes it has happen fairly often.
Just think if you had to deal with cops questioning you over and over again for hours and hours what would you do in the end to stop this even though you knew your innocent? I bet you that you will end up saying oh yea, I did this. Then they all left you alone and start court procedure, bam your thrown in jail.
I know one true story and it happen on movie I think it is called "Just ask my children" Where two teenager (as witness, and has no relationship to the parent) lied to authorities that they saw a parent molested their own kids, so District Attorney believe the witness, and wanted to make themselves look good. So, they questions two poor boys for days and days until the point where they said yeah, they touched me so that they can leave them alone. Parents was stuck in prison for over 10 years. Finally two sons came forward and said everything is hogwash and these events never happened.
For more information http://www.imdb.com/title/tt0280474/reviews
I wouldn't take guilty plea as determinate whether they are guilty as charged.