Several of the methods used for finding exoplanets are biased so that they're more likely to find exoplanets that are both really big (Jupiter-size or bigger) and really close to their star (much closer than Earth)
As methods of detecting exoplanets get better, I think they're starting to find more systems with multiple planets so the average is probably more than 1
Not an astronomer but if I'm not mistaken, we find exoplanets based on the shadows from their respective stars, we can usually tell if a star has multiple planets orbiting based on the amount of light we see, it's just that it's strangely more common to only have 1 planet orbiting each star, instead of the many we have.
Also, 1 exoplanet per star? That seems a bit low? Anyone know why we don't assume there's 8 planets per star?