Droppers facing each other act unpredictably when both are powered at once

What I expected to happen was...:
When only one dropper contains an item, it should be transferred into the other dropper.
If there's an item in both droppers, each should transfer its item into the other simultaneously.

What actually happened was...:
When only one dropper contains an item, it stays in there.
When both droppers contain an item, the items are swapped, most of the time.

Steps to Reproduce:
1. Place two droppers facing into each other.
2. Put an item in one or both droppers.
3. Power both droppers simultaneously, e.g. with a lever/button and 2 redstone lines.