The new breed of super flushers are out-performing the drainage system.
What happens when the 1.6 gallon per flush water closets start flushing too well? That is what's happening. As everyone has cursed out the 1.6 gpf water closets, the manufacturers have been diligently going about their business improving the flushing performance. So much so that the 1.6 gpf water closets far outperform the old 3.5 models.
This has resulted in some other problems. This newer breed of super flushers are outperforming the drainage system. Unfortunately, plumbing contractors are not keeping up. Additionally, in existing systems, this is a real problem when water closets are replaced.