That's a good question. But its not the orientation that changes the strength, it's the loading.
Under tension or compression it doesn't matter it's the same for straight links. However, on your top rear mount the joint is put under a side load (since it is a triangular link) to keep the axle under the track laterally. And under a side load that is trying to pull the "ball" out the side of the joint. If the joint is turned flat, then it is just forcing the ball against the race, not out the joint assembly holes. If that one joint was two seperate joints (a dual triangulated 4 link), then it wouldn't matter.
Also, the bolt itself is the same obviously, but the strength of the bolt varies in how you load it. A bolt in double shear is going to be slightly stronger than a bolt in tension. Although a bolt in single shear is only about 60% as strong as a bolt in tension. In this case, axle wrap puts the bolt in double shear, but side to side loads really try to slide on the bolt, and put it under tension from the mounting tabs trying to bend out. But with it vertical it is under double shear no matter which direction the axle tries to move, side to side or front/back.
Now, it may in fact be strong enough both ways, but it's better to be mounted in the stronger direction for safety reasons.