The 145 is prone to being out of trim, particularly with very light pax loads. Check in agents also aren't trained with any real appreciation of the effect that seating pax in one part of the a/c only has on the trim. It is therefore possible that pax will be required to be redistributed around the cabin to ensure a more even distribution of weight.
Trim has nothing to do with a/c power or pax weights. The simplest way to think of it, is to consider the a/c as a giant see-saw, with the main wheels acting as the pivot point. If the a/c is tail heavy, then it is going to sit on its bum, so therefore it has to be slightly nose heavy. However, if it is too nose heavy, then the tailplane cannot generate enough force at at take off speeds to rotate the nose into the air, and then you end up making a hole in the hedge at the end of the runway. What you end up with is a range of acceptable trim positions where the a/c centre of gravity lies within set safe limits as decided by the manufacturer, so the a/c will not tip over on the ground and can be controlled safely and easily in the air.
As for the tailplane position you noticed, it tends to adopt that position anyway when on the ground.