You are right when you say a TST is either negative of positive, a priori. However its not quite so simple. Here's why: to perform a TST you inject a bit of protein from a TB bug under someone's skin. Two or three days later you measure the bump produced - it looks like the white bump you get after a bee sting. If the bump is more than 1cm across - it's a positive result, UNLESS, the health system in question has decided that a positive result is above 2cm.
Who decides what is the right size? Well, now you have an interesting question. For statistical reasons, the right size has a lot to do with how common the disease is. There are value judgements associated with the "right size", too. That's were studies looking at risks and benefits of false positive vs false negatives come in. In this respect QFT outperforms the others by far.
IGRAs also, in the end, have an arbitrary cut-off point. The indeterminate results are around this point, and are signals for other types of testing unrelated to IGRAs - usually a chest xray and some clinical follow-up. Another IGRA wouldn't convince you one way or another that treatment or discharge from follow-up was a good idea.
Hope this helps, Rabdo
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