just a few KB, i guess it's normal. as long as it doesn't go to zero, it should be fine. Even if it goes to zero, your router will still be working. I've notice that these few items don't work if it happens:
- New firewall rules can't be saved
- New MAC filter can't be saved
- New scheduled tasks can't be saved
Basically all the configs you want to add that need >0kb extra won't be saved.

Anyhow, ROOter firmwares already pre-loaded with all the modules that you might need. Average users will not need the extra space. If you do need the extra space, upgrade to routers that have more memory like 1043ND.

ROOter firmware packs everything in the 4MB flash memory so that users need no extra hard works in order to use high speed 3G/4G internet out-of-the-box. Or, you can flash the vanilla OpenWRT image, which left users with over 1MB+ space on the JFFS2 partition, but require some hard works on the users' side, in order to get it working. Your choice. 
Nice observation!

I think this could be due to how wear leveling, journaling and compression in actions on the JFFS2 file system, where OpenWRT Linux writes its configuration data on (/etc/config).

The Flash Memory used on the router is a kind of Memory Technology Device (MTD), which is a totally different beasts than the traditional magnetic storage we're familiar with. It has limited write cycles (usually a few thousand to 10,000+ writes per block). It requires special file system support in order to use it efficiently, and also to extend it's working life-span.

When users "save" their "settings", configuration data is written to the JFFS2 partition, where "old" data is deleted, and "new" data is written to different blocks. The "deleted" blocks are not actually deleted but being marked as "invalid". That's why users will see a decrease in space over time, due to wear leveling and journaling. But after a reboot, the space is claimed back, where some "recycling" mechanism kicks in.

I'm no Linux guru though. This is just my hypothesis based on my understanding of information on the Internet. Correct me if I'm wrong.