If your regions of interest are grey matter regions as opposed to white matter, you would be better off using the
connectome2tck command. This will ensure that the mechanism used in connectome construction for assigning streamlines to nodes - by default it searches within a 2mm radius circle from each streamline endpoint to select the nearest node - will contribute to the selection of these tracks.
tckedit, streamlines will only be selected if they genuinely intersect the ROI. If your regions of interest are entirely encapsulated within the cortical ribbon (e.g. from FreeSurfer), but streamlines are terminated precisely at the GM-WM interface, there may not be any intersection between the streamline and the ROI, even though a streamline may perceptually belong to that region. That will be why
tckedit is selecting less tracks than you’d expect.
In one of my many development branches, MRtrix3 differentiates between “white matter” and “grey matter” ROIs, and includes a heuristic mechanism that will make the latter behave more like as one would intuitively expect when using ACT. Eventually this will hopefully also include surface-based ROIs. It’s still a while away though unfortunately.