My understanding has always been that septs are rather like tartans... the actual historical association with a particular clan is largely a fabrication. After all, as has already been pointed out, just as members of a single clan would have worn a huge variety of different tartan colors/patterns, it is not as if particular surnames were necessarily part of a particular clan, while others were necessarily excluded. Seemingly, the primary basis for membership was loyalty to the chief (although this was obviously not unrelated to familial descent).

Nevertheless, this does not make the sept relations or associated tartans to clans unimportant. After all, what most contemporary clansmen/kilt wearers are interested in is not historical reenactment, but rather representing their heritage and culture. In truth, the actual historical clan system (as a social structure and governing system) has evolved into something altogether different: the celebration of one's cultural identity. So, while the idealized clan system and uniform tartan does not perfectly match the organic system of history(which obviously changed and developed over hundreds of years anyway) to ask it to do so is unrealistic... even naive. I think we can proudly represent our heritage without replicating or reenacting...after all we are not playing dress up.