Here is why there is confusion: the only time someone is sentenced to jail for smoking pot is if there is a more serious crime they are clearly guilty of, and the prosecutor or judge wants to give them a lighter sentence. Theft or burglary were the most common crimes I came across. Instead of being required to sentence a defendant to a year imprisonment for stealing, a defendant could plead guilty to marijuana possession instead and get a much lesser sentence. So on paper, it looks like they are serving time for drug possession, but in reality, they were let off the hook for a serious crime.
Links
[1] http://townhall.com/columnists/rachelalexander/2015/08/03/no-one-serves-jail-time-for-smoking-pot-n2033738/page/full
[2] http://www.rethinkpot.org/tags/crime
[3] http://www.rethinkpot.org/tags/jail