I believe it's detecting more sources than expected. This can be to do with how efficient observations are (very good so far), how low the background is (higher than expected, but stable), the performance of the telescope in space, and how good the simulations of the data are (we're still working on them!).

I was very impressed with the picture of the Shapley supercluster (see article), which is very overdense part of the local universe, containing dozens of clusters of galaxies, each of those with 100s to 1000s of galaxies. In addition, we took some performance verification observations at the start, and these were really nice: http://www.mpe.mpg.de/7362095/news20191022

The picture is a bit misleading about the number of sources. It's really dominated by our galaxy, as most sources are point-like. You cannot really see the majority of them individually in the full-sky map given its spatial resolution, particularly as we applied some smoothing to the data to help emphasise the galactic structure.

We're also going to get another 3.5 years more data in the survey, which is going to find a lot more sources.