Why was my surname spelled differently across the generations and is there any significance in each variation?

There is no real significance in the various different surname spellings. The variations arise mainly from the fact that, in past centuries, there were simply no standardised 'rules' of spelling. There was also the possibility of an immigration officer mishearing a name when the emigrant first landed on their shores. It's no surprise, therefore, that so many variants occurred, including the issue of whether you're a Mc or a Mac. Sometimes Highlanders moving south would try to anglicise their names with, for example, McCutcheon becoming Hutchison.