I've noticed that when I try to pull storage through a rail it will only pull that amount no less if available.
eg. Storage is set to pull 100,000 of everything will not pull from a storage with 5,000 stack on the other side of rail. But if I reduce the pull to 1,000 of every item it starts pulling out 1,000 units a tick until its under 1,000 and leaves the remainder there.
Normally when pulling directly from storage a pull of 100,000 all will pickup up from 100,000 to 1 block.
Not sure if this was limited to a pull from ship to station but that was where I ran into this while making a cargo loading system. Also tested in single player.