In order to get precision in checking, a lot of computation needs to be done so the compiler relaxes the precision when facing large disjunctions (networks) of constraints.
Dynamic checks are inserted as necessary but a system that relaxes when things get hairy can't guarantee that it will find all errors at compile time.
The idea is similar to Soft Typing of Cartwright and others, but they were thinking of an interactive system, some kind of programmer's aid. If I recall they ran into problems giving reasonable error messages.
