By coincidence the Note 3 I have here does the opposite, reports connecting @433Mbps to the router @5Ghz, but 150Mbps @2.4Ghz. (Not the router, laptop manages 300mbps @2.4Ghz)
That said, it's a phone. There's very few times it will make any practical difference. Downloading a film from the NAS is the only time I even noticed and that was only because I was looking for it :)