A few notes, currently take cover and open fire are broken, to where it affects both your ship and the enemy ship. So if you hit take cover the enemy ship takes 75% less damage as well, or if you use open fire you take 50% extra damage. Also, you can always get extra cannonball damage by leveling grenades.
I believe those high end numbers you're looking for are reduced by a ships armor, and once the armor is gone you see the actual numbers that are listed. Armor reduces the damage your cannonballs deal, so you don't see the listed numbers until armor is depleted. So just running some numbers you provided armor reduces a ship damage received by around 75%.
489 * 100 = 48900
48900 / 2070 = 23.62
100 - 23.62 = 76.38
76.38% Damage Reduction