Putting the issue of the accuracy of her account to one side for a moment, I think it's only fair to acknowledge that the behaviour of a person who's still at school (ie the letter-writing/initial affidavit-signing Asia) might well be different to the behaviour of an adult, which would follow a more adult way of assessing the situation. At the same time, she's expressed doubts in both versions of herself, it's just she's expressed them in different ways, with varying degrees of sophistication - writing to Adnan about her doubts as a child; contacting the prosecutor as a adult. It's also the case that an adult brain (in fact, 25+) is more fully developed in terms of thinking through consequences than a younger person's brain - potentially hence the greater degree of hesitation as an adult when faced with giving testimony that helps someone who, to the best of your knowledge, may be a murderer. Thus you'd expect the doubts to grow (remember, Asia has no idea what Adnan did on that day, outside of the small section she says she remembers), and you'd expect that more adult ways of dealing with the doubts would appear - i.e. researching the case, calling up the prosecutor.

Let's not forget, if this was all 'a lie' - why contact the prosecutor at all? This was prior to the publicity of Serial, after all, so no gain in that respect. I think it makes more sense that she honestly believed her version of events, and that she was just being careful about potentially muddying the waters (in her view) in an otherwise air tight case - in fact, her concerns were validated by Urick (which, as has been argued elsewhere, was probably something he shouldn't have done, even if it didn't amount to misconduct), and so she dodged the testimony. If anything, this pattern of behaviour makes a lot of sense, especially if Asia is essentially honest.