How do product groups and industry groups sit together

Why do people compare product groups and industry groups when they're not necessarily comparable? (GS TMT vs MS M&A, most notably) How do these different groups sit together under IBD? There would be significant overlap, right?

For example, I thought IBD was divided into the product groups (M&A, ECM, DCM etc), then under each product group, bankers are further specialized into their industries. So there would be no overlap, and a banker would call himself, for example, a 'GS TMT M&A banker'. If you know what I mean

So if a tech merger deal comes up, would that deal be given to the GS TMT people or GS M&A people?

If someone could explain how IBD is grouped out then it would be awesome