The Holy Prophet's wives are called Ummul Momineen (Mother of the Believers) because of the Quranic verse which says:

"النَّبىِ‏ُّ أَوْلىَ‏ بِالْمُؤْمِنِینَ مِنْ أَنفُسِهِمْ وَ أَزْوَاجُهُ أُمَّهَاتهُُم‏..."؛[1]

"The Prophet [Muhammad] had a greater claim on the believers than they have on their own selves, and his wives are (as) their mothers."

In this verse, first the Holy Prophet's authority and priority over the believers have been mentioned, then his wives have been named as their (believers') mothers. Thus, "Ummul Momineen" is a title or an epithet that has been derived from the text of the Quran.

It should be noted that the Arabic word "Eman" which means faith or belief has been used in two common and particular meanings in the holy Quran. For example, read the following two verses:

1. The Quran says:

"یا أَیُّهَا الَّذینَ آمَنُوا آمِنُوا بِاللَّهِ وَ رَسُولِه و الکتب الذی نزل علی رسوله‏..."[2]

"O' ye who believe! Believe in God, and His Apostle and the Book which He hath sent down unto His Apostle."

The addressees in this verse are the believers, to believe in God, His Apostle and His book. It clearly indicates that a mere hereditary belief will not be of an avail to any one. This is a clear warning against the blind following of the ancestral belief, or any hereditary creed or faith.

2. The Quran also says:

"قالَتِ الْأَعْرابُ آمَنَّا قُلْ لَمْ تُؤْمِنُوا وَ لکِنْ قُولُوا أَسْلَمْنا وَ لَمَّا یَدْخُلِ الْإیمانُ فی‏ قُلُوبِکُمْ"

"The desert Arabs said: "we believe." Say thou (unto them): "Ye believe not, but say ye, "we submit for faith hath not yet entered your hearts."

The desert Arabs who were Muslims merely for namesake and who had refused to accompany the Holy Prophet on the pilgrimage, driven by the miseries of a famine flocked into Medina with every one old and young of their families and just to get the relief they needed, claimed every help professing themselves to be believers but at heart they were the same disbelieving ones, in God and the Holy Prophet. Thus, we see that the term "Eman" in this verse has been used in its particular meaning. Hence, the meaning of Eman and Momeen should be figured out with the help of the contexts surrounding the verse.

Coming to your question, it should be said that if you look carefully at the verse which calls the Holy Prophet's wives as the mothers of the believers, you will come to the conclusion that the word believer has been used in its general meaning i.e. Islam and that the epithet of "Ummul Momineen" is not different from "Ummul Muslimeen".

For further information, seen (Index: The Phrase Ummul Momineen", question 455 (website: 462).