Digital BW, The Print

Sorry if this is a stupid question,  I've 
tried digging around in search to no avail.
Being a complete newbie means I probably
don't even know what to look for.

I've set up an Epson 2200 with the UT-7 inks
and the cartridges in the LC and LM positions 
have run out rather quickly, while the rest of the inks
are still 2/3 to 3/4 full.  Is it normal 
for  the light gray and light toner to be 
used at such a disproportionate rate? 

I haven't made nearly as many prints as I
expected before running out of ink, but 
I did make several swaps between photo
and matte black so I assume that wasted
a certain amount of ink.

Thanks in advance for any advice.