There are many possible reasons for this. Some marijuana chemicals are converted into their active form by the liver. If a person continues to smoke marijuana, the liver gets more efficient at making that conversion. Marijuana chemicals are also stored in fat and other tissues. In a frequent user, it takes less marijuana to get high than a non-user would require because there are already many marijuana chemicals in the user's body.
Frequent users are also more familiar with the changes in mood produced by marijuana.
Thus they may quickly recognize minor changes caused by marijuana at low doses. The inexperienced user may miss the subtle changes in mood brought about at a low dose and at first respond only to more dramatic, high dose effects. In the non-tolerant user, the peak effect from smoking a joint is achieved in about 15 minutes. Marijuana metabolism in the context of urine testing for cannabinoid metabolite. Marihuana smoking and simulated flying performance.