Unfortunately, the AirPort Express will only "extend" a wireless signal from another Apple wireless router.....an AirPort Extreme, Time Capsule, or another AirPort Express.
The AirPort Express will "join" other wireless networks for use with AirTunes or AirPlay, but the Express does not provide any additional wireless signal when it "joins" a wireless network.
It appears that your AirPort Express has "joined" your wireless network.
I agree that it can be confusing. You might want to let Apple know:
Here is what Apple says in their AirPort Express literature, which I imagine that some users miss:
Extend an existing Wi-Fi network’s range.
If you already have a wireless network in your home and want to extend its range, AirPort Express can help. Just place it in range of your primary base station — an AirPort Extreme, Time Capsule, or another AirPort Express