Not sure if this sheds any light, but there are some regional differences in the pronunciation of initials.
I thought the following post about Mandarin as spoken in Taiwan was interesting, especially the part about
(a) Failure to "juan3she2" (curl the tongue), so "sh, zh, ch" all sound like "s, z, c"
A a result of your diagram, just now I tried to pronounce the zi, ci, si initials as my tongue tip "scrapes down the backs of the middle front two upper teeth" ...... It wasn't a pretty sight.