Understandability here is more of an umbrella term for the system as a whole, cyclomatic complexity is generally only useful for much more isolated chunks of code. Reducing CC can also make the the system as a whole less understandable, moving an inner loop into it's own function for a simple example will reduce the cyclomatic complexity of the outer function yet the complexity hasn't really changed and now there is more indirection for the developer to deal with. It's a good tool for telling you how complicated a piece of code is, but generally there aren't many options to reduce it other than hacking the metric and making the system as a whole more complicated and less understandable.