The free space path loss is higher for 5GHz than for 2.4 GHz. So 5GHz signal get attenuated more for the same disatnce and clients tend to connect to 2.4 GHz if the signal strength is below a threshold based on client's driver algo.
To address this problem Aruba came up with a feature called "Bandsteering". Once you enable this in either "Prefer-5GHz" or "Force 5GHz" or "Band balance" modes, clients connection to 2.4 GHz would be denied initially and clients will made to prefer or forced to connect to 5GHz or client count balanced between 2.4 & 5 GHz.
You can find more information about this on:
http://community.arubanetworks.com/t5/Controller-Based-WLANs/What-is-the-band-steering-feature-and-how-does-it-help-us/ta-p/172154