ISFJ wants to be productive and work until the entire list is complete. It breaks all the To Do items into practical steps, then just plods methodically through them, and will not quit until everything is finished regardless.
NTP actually needs space. It likes to "sit in stasis," experiencing and observing, as if to be free in case something interesting comes along. If forced to engage by circumstances, it will immediately leap into the fray and blitz through the problem. ENTP might be flightier and need to stay focused once the initial excitement dies down; INTP tends to push until the strategy for resolving the problem is resolved and then will quit. The Ji+Pe combo here means that on the outside the NTP will "play and dabble" rather than making a concerted effort while inside the NTP is calculating and thinking. INTPs do things on the surface to feed the internal process, so accomplishing things in the external world is not even the priority except as in how it informs the Thinking process and gives it more to figure out; ENTPs are less about getting to the end of an external process and more in just exploring it in toto.
Needless to say, this drives the ISFJ batty and is seen as lazy.
It also explains why "vegging" on the computer or TV is seen as lazy by ISFJs; if they do it, they are in total "veg-out" time and potentially also not doing their work they've made a list for. However, for NTP, this time when it looks like they are doing nothing, they are actually "working" -- they're exploring and collecting data, and internally processing it where the ISFJ cannot see.