How can't you not understand each other if you guys have the same function and both of you are introverted?
They might have the same functions, but they don't mean the same thing in their positions.
INTJ: with their positions explained
Dominate, 1st)
Ni
Auxiliary, 2nd)
Te
Tertiary, 3rd) Fi
Inferior, 4th) Se
ISFP:
Fi Se Ni Te (you get the point)
ESFP:
Se Fi Te Ni
Applying it to the INTJ
Auxiliary supports the dominant function, e.g. Te supports Ni
Fi (is mentioned to appear child like), however Fi supports Se. (not the other way around)
Ni and Te is conscience, Se(4th) Fi(3rd), I changed the order on purpose, to show the shadow (notice it is ESFP, not ISFP). Also referred to as the unconscious, or the part of your personality that you despise (when younger). Jung said, that to become enlightened, one has to accept that side of their personality. Maturity can help, but a great deal of people never reaches it. His focus was to get people to accept that side, or become conscience and grow, with maturity.
Btw Jung did write the INTJ as Ni Te Se Fe... (I could be wrong here but I think Meyers changed it with decades of observation data) Jung also implied that there are more than the 16 types because of the function orders are different.
I'm only answering, the 4 functions that seems similar at first.
It is an interesting point. I recall Dario Nardi saying that when he did testing of brain patterns on INTJs and ISFPs, as they got older, it was hard to tell the difference between the two of them. They started looking alike.
I would guess, that if both were healthy then it could appear similar, even more so with INTJ and ESFP (just guessing)