It seems to be inherent in the process, the drum picks
up toner which it's going to lay down onto the paper,
the transfer may not be 100 percent and residual
toner remains on the drum. If not removed it would
go round again and produce a ghost image. A
not uncommon problem.
So there is a scavenger blade to remove any
surplus toner, there shouldn't be any if everying
is working perfectly, as for a new cartridge.
Mfr's deem this scavenged toner as unsuitable
for re-use, as they would in the case of an inkjet
if it was scavenged as part of the cleaning cycle.
As toner cartridges wear and age, the scavenger
blade deteriorates, becoming less efficient,
leading to gray diffuse streaks and ghost images.
So ideally it should be replaced when refilling.
Cheap aftermarket refillers may not replace
the blade, or use a non-OEM sub-standard
replacement. 500 pages later problems.