See this paper

https://www.emscience.org/en/article/pdf/preview/10.23919/emsci.2022.0002.pdf

I seems that differential-fed just means balanced. Beware of this paper, not everything in it is well understood by the authors. An unbalanced antenna (a monopole for instance) needs a ground. If you have a large enough ground already then a monopole is smaller than a dipole, otherwise a dipole is smaller than a monopole with ground, or about the same size, if you design your ground properly. There is little point in using a differential or balanced microstrip antenna, it requires a ground to be microstrip. There could be other reasons for using two opposite phase feeds, such as getting the right antenna mode.

You can get a balanced feed in CST by having 2 wires at a port, or feeding 2 ports out of phase and combining them, or using a discrete port.