There appears to be no set pattern to this. Some people simply breeze through immigration & Customs (particularly at Sanford) whereas others have to remove everything like belts, shoes etc. I think EU citizens on their own have an easier time, but if they are travelling with non-EU family, risk being put through the wringer. That is probably why my wife and I, both UK citizens with maroon EU-UK passports, were photographed and fingerprinted. I cannot think of any other reason (apart from our Ethnic background).