Because of what was in the hole with it. Really didn't sound all that good and the vdi numbers were jumping around, except in one narrow direction I got a good steady hi tone with vdi of 85. I thought this needs investigating . Dug a three inch by three inch plug and out popped this 1963 silver quarter. I thought " what made that thing sound that way?" Scanned the dirt pyle and the hole with my propointer and found a wheatie, a memorial penny and a Jefferson nickel in there with it! The ATPro and 5X8 DD had done it again!