The single curved sapphire is cheaper hence being installed in a base models. The cool distortions come as a bonus of owning a base PAM, which is still the quintessence of less is more for many.
This effect on the base Panerai model sapphire crystals is due to them being curved on the outside and flat on the inside. The next models in the range equipped with the p.5000 8 days movements have the double curved sapphire crystals therefore no distortions like on those from base models.