YSK when transporting anything on a vehicle tray secured with a top strap, you should always have the load in the middle of the loading surface.
Why YSK: when strapping down a load while it's in the middle of the tray/pickup bed, the strap is taking the shortest possible path and will stay tight. If the load is off to the side, not only is the strap path longer, but the unequal forces by the strap on the load will want to push it away from the more vertical side towards the middle, ultimately resulting in the strap coming loose.
This may not be intuitive, as evidenced by the amount of loose flapping straps or even straps and buckles I see lying on the road while driving. You can test this yourself by strapping down a box towards the side of the bed - shove the load towards the middle and suddenly the strap is loose. This is just geometry, and is further demonstrated in this diagram, where the strap length with the load in the middle is 1830mm, but when off to the side it's 1933mm.
Keep everyone else safe, and keep your load safe. Strap down your stuff properly.
(And yes this is also avoided by putting a belly strap or otherwise securing it to the side, or have the whole width of the surface filled.)