Keep in mind that this isn't true for every system they've detected, it's just more likely than not given current examples - 909 planets in 355 systems according to the abstract: http://iopscience.iop.org/article/10.3847/1538-3881/aa9ff6/m...

There's plenty of differences from our own solar system out there that also don't fit that pattern. Gas Dwarfs are a planetary type that we don't have in our own solar system, for example: https://en.wikipedia.org/wiki/Gas_dwarf