I am using Google DFP and define ad slot like this:
googletag.defineSlot('/1234567/my-beautiful-slot', [[728, 90],[300,250],[640,400]], 'the-best-div-ever');
What I expect is googletag makes requests with all the sizes provided and its reflected in statistics. However, what I actually see in the stats for ad requests is multiple combinations of sizes, say: "728x90, 640x400", "300x250", "728x90, 300x250" etc.
Why not all sizes are present in all requests? It looks like googletag choses some subset on the client, but Im not really sure and cant find any confirmation in manuals.