This seems to be a limitation of how this is reported by the end device :
prtMarkerSuppliesDescription.1.2 = "Waste Toner Box"
prtMarkerSuppliesSupplyUnit.1.2 = impressions
prtMarkerSuppliesMaxCapacity.1.2 = -2
prtMarkerSuppliesLevel.1.2 = -3
prtMarkerSuppliesMaxCapacity
"The maximum capacity of this supply container/receptacle
expressed in prtMarkerSuppliesSupplyUnit. If this supply
container/receptacle can reliably sense this value, the value
is reported by the printer and is read-only; otherwise, the
value may be written (by a Remote Control Panel or a Management
Application). The value (-1) means other and specifically
indicates that the sub-unit places no restrictions on this
parameter. The value (-2) means unknown."
prtMarkerSuppliesLevel
"The current level if this supply is a container; the remaining
space if this supply is a receptacle. If this supply
container/receptacle can reliably sense this value, the value
is reported by the printer and is read-only; otherwise, the
value may be written (by a Remote Control Panel or a Management
Application). The value (-1) means other and specifically
indicates that the sub-unit places no restrictions on this
parameter. The value (-2) means unknown. A value of (-3) means
that the printer knows that there is some supply/remaining
space, respectively."
Waste toner shows 80% in this case.