There is a problem with HERO in that so much of it is extremely detailed, so detailed that we have come to expect that EVERYTHING will be written down somewhere after very careful consideration. That however is not true. There is no Manual of the Universe. Authors do a lot of stuff as they think fit, and they do not always see fit to detail their thinking or to avoid using terms that are quite usefully described elsewhere.
My bet is, that in this instance, the author felt that the full concentration penalty was too much for his vision of crossbows. He decided he wanted to reduce that penalty and so he reduced the associated cost break to something he felt was fair as well. What he should have written, in that case was (Limited Power: concentration , 1/2 DCV, +1/4). He has used the fuzzy bit of the rules, limited power, which is essentially a bit of carte blanche for GMs to customise stuff to their idea of what is fair and reasonable.
All of the other limitations are essentially examples of what Limited Power might be, what the authors of the rules think are reasonable cost benefit ratios.
