It looks like the frames in that larger image are not consistent in their sizing, if you try and slice by a cellwidth of 187 pixels (which is the first frame width), you don't get a consistent slice boundary. If you slice by 188 pixels, you still don't get a consitent slice boundary and it clearly exceeds the image size (see below). With this kind of variable frame sizing you would need to have a way of sizing each slice area individually which is quite possibly something to implement in a future version.
