My personal theory as to why ISTJs are often misunderstood is that 1) we function in a certain way in relation to the external world, e.g. conscientious, dependable, etc. But this is just a working method for dealing with externalities. It's not a reflection of how we're feeling, what we're thinking or who we are on the inside. 2) The introversion: for me personally, how this manifests is that I'm emotionally guarded with people I don't know very well or don't trust. Other types seem to be comfortable exposing themselves even with people they don't know or don't like that much or can at least make the other one feel comfortable even so. I'm not good at that. I can't help it. It's nothing personal. It's just how my emotional system works. With people I love though, I'm very open and emotional and much more free. I'm also more open that I would be normally when I meet people I naturally click with.
So I think the reason why people say ISTJs are boring, emotionally distant, cold, robots, etc. is because they are identifying the ISTJ too much with the outer functioning. Like a uniform, that's not me, it's just my job, and people who know me better are few and far between. This can be irritating for very image-conscious people to deal with.
So when people don't get a certain emotional response that they're maybe normally used to and expect as normal social behaviour, they might assume that the ISTJ is through and through like that. Just blah and predictable. But it's not so simple.
That said, with age and experience I've understood that people don't like to feel objectified, so I have learned to acquire certain social skills for dealing with people I'm not so close to, I just think it's not an ISTJ's natural gift.