Gustavo , you are right they are the same organs, that does not mean they will grow the same way all the time, as Adair said, their position is critical, a tree doesn't care for your building plans, it will always send more strenght/growth on top and most external shoots (unless you have a basally dominant species/variety). Obviously, those part don't need any help to thicken, we actually try most of the time to restrain them and prevent the thickening of those area that are always growing more than you wish past a given stage of developpement. So when you realize that a part of you tree should be thicker, it's likely to be a lower branch or a more internal part of the tree (typically a branch lacking thickness on its first third of lenght) and that where you need your sacrifice shoot to be so that it ony thicken that area, Those shoots placed there however, even if left free are unlikely to grow a lot (and when you need to thicken an already existing part of a branch, you need a lot of growth), because that's not where the tree wants to grow. After a trimming as soon as growth resumes on the naturally dominant parts of the tree, your internal/low shoot most likely will slow and soon stop growing, that's where the technique is useful, to artificially push this shoot to become a dominant one, only after it has, will it really start to push a lot of growth and thickening. If left to its own you can wait a long time before its "spontaneous" growth will make it become dominant and really increase its growth/thickening rate. So yes they are basically the same organs, but if you think they will just grow in a similar way, YOU are mistaken, bonsai is about going against the natural growth tendencies of trees, energy balance/manipulation. Of course on the tree you showed you dont care, ALL the growth you have now is basically sacrifice to make the trunk grow, when your trees will have passed this stage of trunk growing, you will realize that having strong growth where you need it is not a so obvious matter.