I came across ISTJ description and from an outsider I can see how it seems very similiar. We both express our affection through action instead of words. But the drive behind the action is different. For an ISTJ the completion of an action is the most important and with an ISTP the enjoyment of the action is what matters. When an ISTP is happy we will complete projects, but the second things start to get hectic we lose focus of whats important and instead focus on what makes us happy which is learning new things through experience. When things get tough we dont stress, we figure out how to get things done while still having fun. This can lead to cutting corners, letting certain things go. When things get tough for an ISTJ they focus, plan, and drive towards the finish. Anything that keeps people from reaching there goal can get under there skin and frustrate them. The key is that our goals are different. An ISTJ will see a problem or a screw up as derailing there effort because the finish line is what there shooting for, but an ISTP is just there for the experience so we enjoy all the side roads and detours, screw up can be fun. We may even take a detour that is gonna cause problems because something there looks fun and is worth veering off course for a little while(This is our Ni pushing us to explore everything). In this case sticking to the course is frustrating because you are missing so many chances to experience things.
People who get married young can see the opposite of the person they fall in love with because life has no worries and is stress free. At this point an ISTJ may not be as driven towards the finish line and feel like he has the time to detour without risking his main tasks. An ISTP will actually complete everything. An ISTP still takes his detours, but he has so much time that he actually finishes things through to completion.
I think this is the case with all xxxJ vs xxxP.