The reason for this is that the discs are ordered according to who is performing on them. Thus, all the songs on Volume 1 are sung by the tenor Christian Elsner. On Volume 2 Elsner is joined by soprano Simone Nold, and this disc offers a combination of songs and duets. Volume 3 then gives us performances by alto Ingeborg Danz (including the two Opus 91 songs with viola accompaniment). Indeed, it was only by accident that I realized that the songs I knew best were on this particular disc, not only Opus 91 but also "Immer leiser wird mein Schlummer" (Opus 105, Number 2, which draws upon the same theme as the cello solo for the second piano concerto) and the most sung of all Brahms songs, the "Wiegenlied" (Opus 49, Number 4).
Thus, when it comes to finding a specific composition, the situation is far more problematic than it was in the Brilliant Bach Collection. In that package the listing on the box gave the BWV numbers of the cantatas on each disc. There was no order to the numbers, but at least it was easy to skim for the number you wanted. There is no room to do this on the Brahms box; nor is it really possible to do so, since the songs are not grouped by opus number. In this case, if there is a specific work you want to hear, then you have little choice other than to consult the data CD, preferably with a tool that can search its contents. I might be willing to argue that the inconvenience is a fair price to pay for the bargain price; but, since most of what I have had to say about Brilliant has been generally positive, I figured it would make sense to point out this particular inconvenience out of a sense of fairness!