I'm working on a Trados Studio project containing two files (not virtually glued).
It seems the project statistics does not consider the one that is not being worked now.
That is, the stats do not change upon switching between the project statistics and the current document statistics.
The Project Statistics always takes the current filtering context into account. It can show various statistics depending on how you filter your segments (e.g. by choosing a filter or filtering out/unglueing a document in the 'glued' project.
That is correct if your project consists of multiple documents.