I get the same thing sometimes. Usually it's from nearby iron averaging the signal down. Too make that easier to occur, my ground is iron mineralized, which I think acts a bit as a conductor between the iron and target.
You can sometimes see this by pinpointing and seeing that the target is longer in one direction, though it all depends on distance (I think) between the iron an target (as well as the angle you swing at.)
But in the end, you got to dig lots of those like Jack said. For sure not every one of those is a good target, but I would bet you some of them are.
I would add to make sure you have the trash density setting on high as well as making sure you have your ground (neutral or difficult) set correctly according to the instructions. The latter just seems to add a little bit to your depth from my testing - however brief.