I've worked on COM addins to Microsoft Office products (Word, Excel, Powerpoint, Visio, Project), and each beast is slightly different on the internals and the APIs exposed. This is definitely a case of products created by different teams without solid continuity across product lines.
The hacks we had to put in place to make our plug-in functionality the same on each product still brings back nightmares. My psychoanalyst says the next 5 years of therapy may help...