I fully understand that it is commonly done, and much accepted scholarship is based on the practice, but it can be very problematic to assign authorship purely based on style. In literary circles, similar studies have often been done to try to assign the authorship of unsigned material. With such an approach, many years ago, a number of works were very prominently announced as being by Shakespeare, all of which ultimately had to be retracted (but not until after the scholar in question got promotion and collected a fair amount of money for publicizing the idea). For my own specialty, we know that a certain author wrote many (often short) unsigned items based on purely external (and more reliable) evidence, but stylistically they are not all that similar. One item that is stylistically very much like this same author is generally dismissed because the author himself said it was not his in a surviving letter of unquestionable authenticity. (The claim, however, may not be what it appears to be since it may have been self-serving for other reasons. It is also possible that this author had started the item, which is a review published in a magazine where he had been an editor, and that it was completed by someone else after he had left, meaning that it may be his and not his at the same time.) The bottom line is that these things are complicated, and it turns out that authors, particularly those who write a lot, often write in different styles (or even sets of styles) for different purposes. This difficulty may be more true of people who write for a living, and in a more modern context.

And maybe Paul was just in a bad mood when he wrote the letters that sound less like him. People are not machines, and we are often simply not consistent. The answer must often be that here is the argument (or arguments), but we cannot be sure. If we could be sure, most of the time, many professional scholars would essentially be out of a job (or at least find it harder to publish).

Robert said
The deutero-Pauline letters are not merely stylistically different but also presuppose an historically later church structure and evidence differing theological concerns. 

  

And all of those arguments would be much more interesting than questions of style.