The editor is not WYSIWYG, but WYSIWYM (what you see is what you mean). Especially quotes look very different in the editor compared to the final post.
It's a standard CKEditor, if someone knows how to fix it, it would be great...
However, the next version of IPB (v4.1), coming in October, has a lot of changes and improvements on the editor.
They run that version on their own customer support forum site already, and quotes look normal and work properly there!