15 amp vs 20 amp 10-3 extension cords

Ok, probably should have put this in the shop forum but I rarely go there and I am going to use it on machinery   .  I am running a cord with a drop quad box over my shop table.  Tired of tripping over cords.  About 40' from a dedicated 20 amp box at the panel.  want 10-3 to minimize voltage loss.  Might have three tools plugged in but only would have one running at a time.  Now, if I run a 10-3 cord with the t plug, it is 30 amp.  If I get a dedicated 10-3 extension cord with a single outlet, I can get 20 amp, however, if I get a 10-3 extension cord with a triple outlet, it is 15 amp.  Can some one explain this to me since the conductors are the same, the distance is the same.  Thanks.

15 amp vs 20 amp 10-3 extension cords