There have been over 14,000,000 past month users for 3 straight years now, so you are correct. BUT-- those are the 3 highest recorded numbers since 1985. According to the NSDUH surveys, the last 3 years have seen the highest past month mj use since the office of Drug Czar was created in 1989!
That's what the chart indicates.
I say it's due to lax enforcement (marijuana arrests lowest priority), the move towards decriminalization, and the recent medical marijuana laws. All of these changes are lowering the perceived risk of marijuana, while the perceived risk of other drugs remain relatively flat.
You reap what you sow.
ONDCP was created in 1988. Everything on the charts prior to 1988 are meaningless. ONDCP has not met its goal of reducing drug use. The War On Drugs is a failure by ONDCP's own standards and mission statement.