Why does adding a transition TAKE AWAY from my total
time???
Some transitions affect the total length of the project while others do not. Don't miss what Smtr said, that the Overlap transition is different than the Cross Dissolve transition in this regard. In your case, the Overlap transition may be what you should use.
The Cross Dissolve transition borrows 2 seconds from EACH side to build a 2-second transition. Put another way, it borrows 4 seconds total to build a 2-second transition. That's a net loss of 2 seconds to the total length of the project.
The Overlap transition, on the other hand, borrows 2 seconds from only the right side to build a 2-second transition. There's no net loss.
A Cross Dissolve transition must behave like that so both clips continue to zoom or pan during the transition. An Overlap does not continue the pan or zoom on both sizes, so it doesn't have to borrow from both sides. Essentially, it plays 2 seconds of the right clip on top of the last frame of the left clip.
To see the visual difference, import two photos with a zoom or pan — imported with the Ken Burns checkbox ON. Duplicate both clips on the Timeline, then apply a Cross Dissolve to one set and an Overlap to the other set.
Karl