At first I was going to say some poor old, tired monk on a cold, drafty night made a slip. But no not really.
The difference is mostly discussed on the Eastern Catholic Churches websites (as best as I can determine) so, of course, the perspective is not as objective as it should be. However, this is what one site said:
"The Creed as formulated at Nicea in 325 ended with the words “And we believe in the Holy Spirit. “ When the Council of Constantinople met in 381, it officially adopted an expanded version, the Creed (without the FILIOQUE) as we have it today. This expanded version is in fact older than 381. It was the Baptismal Creed of the Church of Salamis, on the island of Cyprus, and is quoted by St. Epiphanius of Salamis in 374 in his ANKROTOS (see below). It was apparently a reworking of the Baptismal Creed of the Church of Jerusalem, which in turn was a reworking of the Nicene Creed.”
Ref: http://www.thefathershouse.org/creed/filioque.html
The Eastern Catholic Churches don’t like the expanded version (“and the Son”) because 1) they believe the Holy Spirit comes from the Father and not the Father and Son; 2) the expanded version was never agreed to by the whole church; and 3) they believe this insertion is untrue since everything comes from the Father.
Western Catholics, Eastern Orthodox and Protestants Christians don’t have a problem with the Creed since we look at the Trinity as being co-equal and co-existing (verses God the Father is head of the Trinity and everything proceeds from Him).
One has to keep in mind that no one has been able to clearly explain the Trinity or how it works so to me all this fussing about the working of the Trinity IMHO is pointless. But, be that as it may, the Nicene Creed and the insertion of the phrase “and the Son” is apparently is a hot button between the Eastern and Western Catholic Churches who have distinctive views on this creed and their theology of the Trinity.