If your router is a dual band version, this means it can switch between different frequencies(2.4gHz and 5gHz) to maintain the best connection. This switching isn't always the best option for all network enabled devices as they an sometimes not realise the rest of the network has switched.
For this reason, the Air is not compatible with a 5gHz network.
Whilst the Air can connect to your router and the network as a whole, it does not know when the frequency changes from 2.4gHz to 5gHz.
Restricting to 2.4gHz means that any device can easily find the network and maintain connectivity as there is only one type of frequency available.