Two points:
The Explora guns were more heavily built than a standard shotgun (as were the Holland 'Magnum' paradox guns). They undoubtedly operated at higher pressure than a Brit shotgun at that time. Perhaps someone here can post actual figures, or an appropriate reference. We keenly await 'The Book'!
Also, there is some evidence to support the notion that the Explora LT-Capped projectile was significantly lighter than 730 grains. Graeme Wright discusses this in the Paradox chapter of "Shooting the British Double Rifle". Has anyone pulled one to find out?

Hope this helps.