Online site doesn't have this as on deal, so looks like it's probaby that one store you were in. Online, 8 pack is £21.50 or £12.50 for 4. The online sites for major retailers like Sainsburys normally mirror what's in store, with some online exclusive stuff too.
It might be a case of over-production by Gillette during the course of last year and they now need to shift the stock that's been gathering dust in their warehouses.