Arengius has always followed that freemium pricing model (one free version and a paid version with more stuff), it was a bit of an experiment with monetization models. My guess is that it shows up as "acquired" because you downloaded it back when it was new and you technically paid enough for it (since the base cost is 0) - I've noticed it acts weird in the seasonal sale backend as well and gets treated simultaneously as free and paid depending on which menu you check.