campervan camping charges

VERY generic query here about camping in one's campervan at festivals where one can park adjacent to one's tent otherwise eg like cropredy, H4TM, Tangled Roots etc.

Its not unusual for a festival ticket to include tent camping, but to charge extra for use of a campervan.

I'm wondering what the issues are particularly regarding use of a campervan which would incur extra costs for the festival organiser here ... as basically a decent sized tent is no smaller than a campervan, and (as above) the vehicle whether a van or a car would be parked in the same area as well.

Please note this is not a "whinge" about campervan charges over tent+vehicle... I'm genuinely intrigued as there must be a sensible reason why campervans would need an extra charge.

anybody any ideas?