Could you say what is postposition in chinese and why it's different from preposition
I think you are making a fundamental mistake. Your conscious or otherwise presumption is, Western Grammar is correct and it must therefore apply, without reservation, to Chinese. Words are placed in categories such as 'preposition', but words refuse to remain in their categories. This is true in Chinese and English.

And dear old Aunty Wiki blathers on about it, as if she knows by divine right.

What about 前？Is 前 a preposition? Well, that just depends on how it is used and who is translating.

会议向前提了。
meeting to front/forward carried
The meeting has been brought forward.

我只读了前几行。
I only read 了 first/front few lines
I only read the first few lines. 

他走前把门关上了。
he go before 把 door close up
He closed the door behind him.

前不久
before not long
not long ago

就在圣诞节前
just in Christmas before
just before Christmas

在后的, 将要在前。在前的, 将要在后。
So the last will be first, and the first will be last. (Matthew 20:16)