Thanks, the transom angle is what I was overlooking.
Well, it actually shouldn't matter, the angle of the transom, and you are mostly correct that once everything is aligned, the gimbal bearing is straight, barring manufacturing irregularities, damage or rot.
If the transom angle is steeper, the front of the motor ends up being higher. The angle of the motor to the transom plate is supposed to be exactly the same, 13º, in every installation. However, if that bearing was a fixed bearing, you could never get the alignment bar in unless the engine was perfectly aligned already. And if there were any flex, the pressure on the drive shaft would be high, and vibrations could damage transom plate.
Here's one engine drawing where you can see the angle between transom plate and engine is always 13º.
Rick