The original VHS was a 15 so it was probably something to do with extra features that featured something worthy of an 18.
This has been the case with films in the past that had a trailer reel featuring other rated 18 films.
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MIKE: I've got it! Peter Cushing! We've got to drive a stake through his heart!
VYVYAN: Great! I'll get the car!
NEIL: I'll get a cushion.