warning: very tired and therefore, might be rambling

Professional: Any one who practises an occupation, whatever that might be.
However, the training that one undergoes prior to practise might not necessarily be classified as such. Professional degree programs impart a certain specific skill to their students. Such programs are geared at adding to the task force of a certain trade. Graduates of such programs practise just that specific skill and are largely specialised and restricted to their particular training, viz., teachers, doctors, engineers, lawyers, architects. In these cases, the term 'professional' thus becomes doubly imbued with its connotation. In a sense, since these people receive skilled training in a particular trade, they could also be referred to as traders.
When training imparted is more varied and less restricted in its scope, the opportunities for practise are equally so. And therefore, whilst each one of such graduates becomes a professional in whatever he or she chooses to practise, their training or degree might not be commonly categorised thus.
I don't know that I am right; this seems like a likely explanation for the confusion, if any.