I’ll take the second question first.
Changing around what inks are used to create colors is actually what color management is all about. I think we all get used to the idea of color management changing RGB values into CMYK values at the printer, but the same thing happens with CMYK input as well.
Your well-made CMYK profile is designed to adjust everything going to your printer to be as accurate as possible. But my guess is that your document CMYK is based on something else, maybe SWOP or a GRACoL profile? So you’re still going to need a transformation of how color is made in order to faithfully transmit your “document-space” CMYK into “printer-space” CMYK.
A famous quip around the office here is “CMYK values are not colors - they are just numbers.”
Here are some links to some of our newsletter articles:
Myth 11 talks about this from an RGB > CMYK point of view, but the same thing is applicable in CMYK > CMYK.
Myth #25 has good background information on how CMYK to CMYK transforms work:
Concerning why your output is different on your identical printers -
For diagnostic issues like this, ColorThink is a great way to get some answers. If you can email me your measurement files and the profiles you made for these two, I can take a look at them and report back what I find in this thread. ColorThink allows us to look at what’s actually happening to the color in detail, and sometimes that can give us clues as to where to go to find the problem.