You are correct, on some boats it is darned hard to get the Trim Tabs all the way outboard where they should be. But they really should be as far out as they can. If the Trim Tabs are too close to the centerline, like the boat in the photo, when you try to correct for a list you will also get bow down input from the lift being so close to center. Having them outboard "uncouples" the side to side component from the fore to aft component of their functions.
When an installer comes across a transom that is challenging, they should contact their Trim Tab manufacturer (or me
) to see what options might make it possible to install them in the best possible position.
Chris196 is exactly correct regarding why the ski boat manufactures us one Trim Tab. Notice that they call it a wake plate not a Trim Tab. There are some watersport boats that are actually using three Trim Tabs, one in the middle and two conventionally mounted ones to give maximum control of wake and attitude.