According to this answer to "what gear should go in the brain (top most compartment)",
the brain compartment usually ends up becoming the knick-knack / catch-all pocket, for small items items that get frequently used
However, I noticed that the zipper of the "brain" compartment almost always faces backwards, towards the wearer and the straps, rather than forwards towards the other zippers and pockets on the front of the pack.
Typically when I take my pack off, I set it down or against a tree, with the straps facing away. This allows me to open the pack and grab something out of the main compartment, and it makes it easy for me to access whatever water bottles or other equipment I've clipped to the outside of the pack.
But with everything else facing up, the brain compartment zipper is facing down. All of the "small items items that get frequently used" will fall out if I open the brain compartment zipper while the pack is in this position. So I end up having to turn it upside down to get things out of the brain compartment. The pack is usually heavy enough to make this rather inconvenient.
For a long time I've been wondering about the purpose of this design. What am I missing? Are people able to get stuff out of the brain compartment by reaching backwards while wearing the pack? I know that as for me, my arms are not nearly flexible enough to do this.
I asked the owner of the local military surplus store and he said that the reason for this design is so that the brain compartment zipper doesn't come open when the wearer of the pack is crawling through underbrush. I'm having trouble believing that most backpackers spend enough time crawling through underbrush to make this an important design consideration. Does anyone here have a more believable explanation?