Pack: SkyFactory-4_4.2.2
mod version: 0.0.24
Single Player world.
Alternate Controls (altMode) is on in the client settings.

Single left clicking with an empty hand on a crate/barrel usually returns two stacks. On occasion it returns one stack (as expected) instead.
Sneak + left click with an empty hand usually returns two items, on occasion only one.
When hand is not empty, returns one stack/item, as expected.

Whenever 2 stacks/items are returned, the box/barrel gui is updated twice, both times decrementing the count inside.