Digital BW, The Print

This is weird......
I normally get maybe 30-45 8x10 prints out of Ut2 color cart. The eboni 
cart lasts maybe 5 times as long or more, because I do a lot of prints 
on costco glossy, but I have this one color cart in my 1280 right now 
that I swear has to going 80 prints. That is so wierd. After about 40 
or so I checked the levels, and it was almost out. Basically empty, so 
I got out the box of new carts, and made a few more, then a few more, 
then I did a whole photo album on costco glossy for my brother-in-law 
as a gift, all the time ready to rip off the taper from the new cart. 
Then I checked the level again and it's significantly more full than it 
was 30 prints before, which was after about 40 prints! Weird. OK by me 
though, but I wonder why that is. It makes me think that maybe there is 
still a lot of ink left when the light goes on. How many would be about 
the max on a full color cart. Most of the pictures were from a wedding 
that I did, which were of various densities from light to dark. Lots of 
black tuxes, but also white dressed etc......Any ideas why this is 
lasting so long?
Doug M