Detectors are all different, even the same models will vary from machine to machine. Components change values with time, and how it has been stored can affect it's components in a dramatic way if it has spent a lot of time in the hot trunk of a car or attic of a building. Some are just naturally "HOT" when they leave the assembly line, others of the same model can be on the "DUD" side.
Deep machines get the deep treasures, but also get a lot of deep trash, digging time and your objectives must be taken into consideration. Beaches are huge and need lots of depth, along with large coils, PI's tend to trump on beaches. Relics can also be very deep, and VLF's tend to be the preferred machines. In iron infested sites a good old TR is hard to beat, and on and on...
Keep trying them out and keep the one's you really like that actually match your own style of digging. There are lots of good machines out there, life is short, so try to taste as many as possible. You might already own the best machine for you, but you won't know until you try out some more.
Good luck & enjoy yourself!