Why do some inkjet printers insist on using all inks for cyan,magenta, etc.?

[Mac-S]

I think your problem is this:
100 % cyan in your desktop application ( photoshop...) is not the same as 100 cyan on your printer.
I find that this is easiest to see in the yellow areas. Very often you see meganta drops in there. Because your printer / RIP is trying to simulate the yellow of you Photoshop ICC.( A warmer yellow )
If you can disable all icc usage in your RIP the problem will go away. This is offcourse not the right solution. Good profiles is as always the answer.

[imagesetting]

Something is adjusting the outout colour for you, RIP, Printer software, etc, the ink in the cart is 100% of its colour.
As explained you will get very light tints of other colours in your primary patches as it adjusts colours to fit a profile, for example most inkjet yellows are far more vibrant than Litho yellow, so profiled printers will add tints of other colours to dull it down so it matches when trying to match a litho print. Some inkjet inks can be far too saturated - and hence the speckles to adjust by just a few % - or what ever value is deemed necessary to match whatever target is measured.

Exhibition Display Services, Cromalin Proofing & Prepress

[Enrique]

Thanks guys, you've given me a lot to go on I'll see what happens.