iOS Instruments Allocations -- Find Out What/Why Memory was Released -

iOS Instruments Allocations -- Find Out What/Why Memory was Released -

i have spritekit game play song @ slow tempo , @ fast tempo. force gameviewcontroller slow tempo. after level complete, force gameviewcontroller fast tempo. both of these screens have same image info (in skspritenodes), 60 mb worth of backgrounds essentially. can see in first image:

when switch fast tempo song, of imageio_png_data allocations go away! can still see images!

questions

why/how happening? in general, how should find out why memory released (not allocated)?

ios memory-management uiimage sprite-kit instruments