sprites packed by sprite packer are worse than a grid

I’ve got 860 64x64 sprites, auto packed with the default packer policy. they end up in 2 spritesheets. if i tightly pack a sheet manually with GetPixels/SetPixels, i end up with 1 sprite sheet with enough room left over for 164 more sprites.

Why? is this because of default padding or something?

Padding.