The AirPort Time Capsule broadcasts both a 2.4 GHz and 5 GHz signal using the same wireless network name. The Mac will automatically connect to 5 GHz if it is close to the AirPort Time Capsule and connect to 2.4 GHz if it is room or two away from the Time Capsule.
This makes things simpler for most users, rather than having to choose networks or guess which network to connect to. That's likely why Apple uses this as the default setup for the AirPort Time Capsule.
There is an option to assign a different name to the 5 GHz network in AirPort Utility, so if you enable this, you will see two separate networks being broadcast.....a 2.4 GHz network with the normal name and another with "5 GHz" added to the name.....so you can "point" your Mac to the network that you want to use.
The downside to doing this is that if the Mac is a few rooms away from the Time Capsule, that you may try to have it connect to the 5 GHz network when the 2.4 GHz network would be a stronger, and likely faster signal. That's because 2.4 GHz signals are much stronger than 5 GHz signals.
Try powering off the AirPort Time Capsule, waiting a few seconds, then power it up again to see things straighten out. Might be a good idea to restart your Mac as well.