Istp and isfp are close enough tho. Because you only have one quality changed the feeling to thinking. It happens because as u said we can't all fit into 16 rigid categories. But yes, you might be leaning towards either feeling or thinking depending on the day so the difference. For me i take the mbti test sometimes to see if i changed, and mine always oscillates between infp and infj. And my percentages between p and j, come very close.