There was some chat about confusing car classes in the comments of this post on the Avis UK blog earlier this year. Basically Avis follow the European standard for ACRISS codes (pdf) which, despite being redrafted in the last decade, still has many loopholes. Vehicle cost can still be a particular cause for skewing classes, especially if you find a high priced compact like a BMW 1 Series.

And yes, the definition of "executive" is certainly up for question, especially when you're house hunting in the UK