I found the problem. The original document with SimSun fonts had been created on a server that did not have the SimSun fonts installed. When my colleague opened the document on his PC (which also does not have the SimSun fonts installed), the TOC page numbers were OK, apparently just like on the server. But when I tried to open it on my PC (which does have the SimSun fonts installed), the TOC page numbers were wrong. Mystery solved.