Previously, LEO altered the Two-Shot effect from its vanilla behaviour (adding a single projectile) to doubling a weapon's base projectile count. This change goes back a couple of years. Back in the day, when LEO was still part of old ECO and its predecessors ([Legendary] Crafting Framework), there were two effects available, the vanilla variant with a single additional projectile and a separate one that provided a multiplicative approach. Due to popular demand and in order to simplify things I removed the separation and made the vanilla effect multiply projectiles by 2.
This isn't necessarily the best approach for every kind of weapon or use case, thus LEO features both variants from v1.9 onwards again so that you can choose on a per-weapon-basis.

Here's why you'd want to choose one or the other variant:
The more projectiles you add to a weapon the less damage each projectile deals because the overall gun damage is equally split across all projectiles. That's why the Two-Shot effect also ups the overall damage by an additional 100% so that the change in projectile count wouldn't result in a net loss in damage per projectile on guns with just a single projectile but instead each of the two carry the original gun damage. In case of shotguns, where there are already multiple projectiles, the original Two-Shot would result in a net gain per projectile but since it already is way harder to hit with all of them this is a fair trade-off. But if you now multiply the multiple projectiles, then chances are, you're missing even more of the projectiles because Two-Shot also makes accuracy and recoil worse. So, you'd lose even more damage.