You're welcome, check-feel,
The Dissolve tool, simply said, does not merge; it typically takes away definitions in a certain area of the model. Merging, from my point of view, would be more about creating a connection between or among polygons while not reducing the details.
The typical result of the Dissolve in Forger is an N-Gon, which has some advantages during modeling, but anything after modeling might produce questionable results. Typically, the mesh is triangulated inside the render engine, which might not be how it is preferred. N-Gons are polygons with more than four points to define the boundary edges. If two polygons are dissolved in an N-Gon, the amount of boundary (island) points does not change.
(Many three or 4-point polygons in a group are not an N-Gon.)
I hope that helps.