The language and tool set is a big factor here. If your tools count imports, declarations, etc., your coverage may be much lower than 80%--but still may test everything that's important. Statistics lie. What do you hope to get from 100% coverage?

The coverage percentage which can be reached depends on the programming language and the tools used to measure the coverage. Reaching 100% of all non-dead code with disciplined use of TDD happens naturally.

The problem is covering those lines which are required by the language syntax, but will never be executed. For example in Java, the private constructor of a utility class which is meant to be never instantiated, or a catch clause for a checked exception which will never be thrown (for example ByteArrayOutputStream.close() is declared to throw IOException even though the method's implementation is empty and even its javadoc says that it will not throw an exception). Also some code coverage tools may calculate the coverage incorrectly and give false positives (for example reporting abstract methods and interfaces as not covered).

Also remember that 100% line coverage does not mean that all execution paths and all states have been tested. To really cover a class, the programmer needs to write tests for all interesting states in which an object can be and for all transitions to other states. AFAIK, there are no tools which can measure that.