At a guess, 'imaginative' comes from imaginatus, the past participle of the Latin verb imaginor, while 'preventive' comes from praeventus, the past participle of the Latin verb praevenio. In other words the -at- in imaginative has always been there, while the -at- in preventative is a modern innovation and therefore to be resisted to the last drop of the last prescriptivist's blood.

Bingley

Bingley