The thing with MBTI is it looks at cognitive functions, not actual behaviour. People can share cognitive functions but behave differently. Also when people look at MBTI for the first time they look at the dychotemy of the letters (I/E, S/N, T/F and P/J) when they should be looking at the 8 cognitive functions (Si, Se, Ni, Ne, Ti, Te, Fi, Fe). It's hard to give a simple explanation.
I shall try use you and Tully as an example (based on the assumption that you and Tully were 'typed' correctly). Tully being an ISFP would mean her top 4 functions in preferential order are Fi (introverted feeling), Se (extroverted sensing), Ni (introverted intuition) and Te (extroverted thinking). Now you as an ISTP means your top 4 functions in order are Ti (introverted thinking), Se (extroverted sensing), Ni (introverted intuition) then Fe (extroverted Feeling). Even though there is only letter difference between ISFP and ISTP there is a big difference between the functions and the order they are used.
I don't know if I explained it well, it gets quite involved......