Why the proposed Vigilance corvette uses a 57 mm gun

What is the case for the 57 mm gun on Saab's Vigilance corvette design for Canada?

Canada already supports the 57 mm, so commonality can justify it even if the 76 mm offers a heavier projectile.

For Saab's proposed Vigilance corvette, the case for a 57 mm gun begins with Canada already having that calibre in service. A 76 mm weapon is preferred here for its heavier projectile, but an existing, viable 57 mm system is considered an acceptable choice. Commonality supplies a reason to retain the smaller gun. The spoken projectile-weight comparison is tentative and corrected during the answer.