[Perhaps this is a "which came first - the chicken or the egg?"-type question - but I'm left quite puzzled why anyone would describe things in this manner.]
Why would anyone (or any material for that matter) describe dialects/topolects in terms of Mandarin? Most topolects/dialects would certainly be much older than MSM (Modern Standard Mandarin) - as a, relatively, new 'invented' language.
e.g.: 广安方言与民俗词典 describes a bunch of 广安方言 in terms of:
even going as far to describe some words as:
here's some examples:
声母变异: “敲” kao1 - “qiao1”的音变。指打，击。
韵母变异: “眉” mi2 - 眉“mei2”的音变。如“眉毛”、“峨眉豆”等。
自创音节: “□” pie4 - "指破烂，坏，不好。常写成“孬”。
I highly doubt that these are "变异" seeing as these pronunciations outdate the modern "standard" ones - so why would anyone go to the trouble of describing it this way? besides the fact that MSM is the, now, "standard"?