DFs differ in Powers, but it's what's common between them all that defines what those Devils are,
They all are Rejected from Afterlife, when a DF User Dies their Devil doesn't travel with them to Afterlife (Except Brook),
They instead remain tied to this World & look for New Host,
If Imu's Power is "The" Devil, then He probably is in charge of such Cycle & Distribution of DFs, (He is like Punk Records of DF Souls)
In other words, when a DF is Created, it must be eaten by Someone & when He Dies, that's when Imu can Claim that Power & then Recycle it,
It's also why his Summoning Circle appeared in the Sky, Imu basically blocks those Devils from going to Afterlife & Captures them,
He allows all DFs to be Recycled, while ones that Interest him, He keeps for himself & his Knights (Gorosei's Yokai Powers & Cerberus are probably Old DFs that Imu decided to Claim instead of Recycling)
This means only DFs that avoided Imu are those that were never eaten or their Users didn't Die yet,
This is why Oda made sure to tell us that Luffy's DF & Loki's DF were never Eaten before, it indicates that Imu never captured them.
Also, this hints that Brook's DF might actually be Anti-Imu.
