It's less that and more that the lightsaber melts the bullet, but for the most part (especially with large caliber bullets) the melted metal keeps going. Now, with more specialized bullets you could possibly have it knocked out of your hand. For example, a beskar or phrik bullet would do that. Now, there are cortosis bullets that will deactivate your lightsaber, but those won't knock it out of your hand to be honest.
In general, slugs are much harder to block with a lightsaber both because they're physical projectiles and not blaster bolts but also because they move far, far faster than blaster shots do. Blasters you can follow and deflect, slugs are impossible to see with the naked eye when fired.
That said, if you have a marksman or expert gunslinger aiming for the HANDLE of the lightsaber, then that'll knock it out of your hands, too.
Just my two bits.