This usually happens because of different smoothing groups on joined surfaces. Think of a cube for example: it really only has eight vertices, but the hard edges would require separate smoothing groups to make it render properly (and not like some weirdly smooth-lit blob). I don’t know where, but somewhere in the chain of editing models, exporting them, and importing them in TU, these vertices are split to make separate surfaces based on the smoothing groups. So, in the case of the cube: there are six separate squares of four vertices that make up the model, which makes for 24 vertices instead of the 8 “real” vertices.
That is my guess at what is happening. So take a look in your model at how many hard edges there are, as opposed to smoothly shaded surfaces.
Edit: it looks like this phenomenon is also covered in the article that CarbonCopyCat provided.