The guidelines were introduced to try and ensure that, for a broadly similar offence one would receive a broadly similar sentence. There will always be differences due to individual interpretation but these should be minimised.
It is possible for sentencers to go outside of the guidelines...
Because most of us know that such a relationship, as well as being illegal, is abusive and, even if we are attracted to a much younger woman, there's an age below which society has decreed that a physical relationship isn't allowed.