Why do the native Mandarin speakers pronounced “r” differently?

Answer 1

To pronounce R , tongue needs to work as the tongue sound which way difficult. In addition, you still need your teeth and lip to associate.

Same as you noticed that some Chinese people have the some problem, who probably live in southern China.

Answer 2

From my personal experience as a native Mandarin speaker, basically (r) won't come up independently in Chinese/Mandarin because Chinese/Mandarin only has the sound of (er) and (ri) in Zhuyin and Pinyin. Maybe thinking about it is not either vowel or consonant in English, so it probably makes native Mandarin speaker pronounced differently.