Just to be accurate, Epson was sited in a class action, and they agreed
to a settlement, so the issue was never actually at the point where
anyone won or lost, however, Epson did agree to pay a sum of money to
the lawyers and users within the US (only).
No as to how the cartridge levels are determined. The process is
subtractive, from a full Epson cartridge. As the printer is used, ink
amounts are subtracted for cleaning cycles, purging cycles, and
printing. These numbers are conservative so that up to 10-20% of the
ink is left in the cartridge when it reads empty. Further, if the head
or some nozzle within it are clogged, that is an unknown, but it is
still assumed that ink is used up even when it isn't.
So, even if the heads was fully clogged (for any color) the ink monitor
would still subtract ink amounts for cleaning cycles, "printing" and so
on, and will eventually read empty, when no ink ever left the cartridge.
Art
If you are interested in issues surrounding e-waste,
I invite you to enter the discussion at my blog:
http://e-trashtalk.spaces.live.com/