Placekeys Found in Advan, not in POI Dataset

I'd like some clarification around discrepancies in placekeys between Advan and Safegraph.

I am compiling a list of first visits between Advan and Safegraph data, the first time a visit count shows up in the data for each placekey. Initially I used the comprehensive POI dataset, since Advan is meant to only include places within that list. However, I found that there are placekeys within Advan that are not in the comprehensive POI data (the dataset with some 20 million observations.) I have searched through the documentation, but have not been able to verify why there is this discrepancy. Could someone let me know why I am seeing this?

Further to this, the parent placekeys in Safegraph do not match the parent placekeys in Advan. I have verified this for individual placekeys that match in both datasets, but do not share a parent placekey. Are the parent placekeys assigned separately between the datasets?