I noticed that some manga's (especially the evil flowers group manga's) have chapters plus letters.
For example: absolute witch
12a, 12b, 12c, 12d, 12e, 12f, 12g 12h and chapter 13 even went to 13i

So I wanna know is there a reason for this annoying way of numbering the chapters?

Cause they're splitting up one official chapter (as defined by the book or magazine or author) into 8 parts. It seems to happen especially with manwha and really long oneshots.