Pronunciation of -o v. -uo

[Jose]

I am not sure about the phonetic difference between the syllables:

bo, po, mo, fo

and

duo, tuo, guo, kuo, huo, luo, ruo, nuo, cuo, chuo, zuo, zhuo, suo, shuo

To my ears, the way northerners pronounce the first four syllables sounds more like "buo", "puo", "muo", and "fuo". I have never read any good explanation of the phonetics of Chinese, so these are the sort of things I've ended up noticing after hearing lots of people speak. From what I hear, it seems to me as if all the syllables above are the result of combining an initial consonant with the "wo" final.

But then, why does pinyin (and zhuyin) use plain "o" in the first four cases? I suppose there must be a reason. Is it wrong to pronounce "mo" with the same vowel as "nuo"?

[HashiriKata]

The finals here are all just one and the same. This is not the only thing unexplained in the syllable chart. I do sometimes find explanations for the irregularities of the spelling but in the end I don't think they really explain anything

[rmontelatici]

I am not sure about the phonetic difference between the syllables:

bo, po, mo, fo

and

duo, tuo, guo, kuo, huo, luo, ruo, nuo, cuo, chuo, zuo, zhuo, suo, shuo

There is no phonetic difference. The spelling is not regular.

There is an explanation: pronuncing labials (like b, p, m and f) implies having a mouth configuration which is close to the one for the "u" sound.

Thus strangely enough b, p ,m and f are considered to include u.

Quite confusing actually.