Well, there is some ambiguity around the meaning of the phrase
τοὺς πτωχοὺς τῶν ἁγίων, although the correct choice appears to be mostly a settled issue among modern translations and commentaries. To define our terms:
The phrase could be interpreted as:1,2
- "the poor saints in Jerusalem"
- An attributed genitive construction: the head noun τοὺς πτωχοὺς functions semantically as an adjective modifying τῶν ἁγίων.
- "the poor among the saints in Jerusalem"
- A partitive genitive: the genitive τῶν ἁγίων denotes the whole of which the head noun τοὺς πτωχοὺς is a part. This is a natural choice for English speakers who would like to gloss the genitive as “the poor of the saints.”
- "the poor, that is, the saints in Jerusalem"
- An appositional construction: the basic meaning is to indicate an equative relationship between the two substantives.
Each of the first two renderings shares the implication that an economic sort of poverty is in mind. The primary difference between them is that the first suggests that all of the Christians in Jerusalem were included as the recipients of the collection and were considered "poor." The second indicates a subset relationship, but still most naturally denotes economic poverty.
The last rendering suggests that "saints" is a restatement of "poor," in which case they should share a similar scope. In this formulation, "poor" is likely a theological rather than an economic designation. Herein, I suppose, lies the explanation given by whatever commentary you were reading. The same term is used with a theological nuance elsewhere (e.g. most obviously, Luke 6:20).
For the reasons outlined in other answers and particularly given the context of a money collection, the economic reference seems more likely. Between the first and second options I have listed with that implication, only the KJV and Geneva chose the first. Most more recent translations (NIV, ESV, NASB, NRSV, etc) chose the second. Commentaries seem to agree. In addition to Moo who makes this conclusion after laying out the options I have summarized, the Pillar commentary states simply, "The genitive τῶν ἁγίων is partitive."3
Douglas J. Moo, The Epistle to the Romans (NICNT; Accordance electronic ed. Grand Rapids: Eerdmans, 1996), 903-904.
Daniel B. Wallace, Greek Grammar Beyond the Basics: An Exegetical Syntax of the New Testament (Grand Rapids, MI: Zondervan, 1996).
Leon Morris, The Epistle to the Romans (PNTC; Accordance electronic ed. Grand Rapids: Eerdmans, 1987), 525.