I agree with Dimitri, this seems to be a QA issue. If the color is correct on the front, & the proofs were showing the same color font & back, you should be able to expect a reasonable match be tween the front and back. The only possible for variation might be the finish on the sheet, (i.e one side dull the other coated) Do you have a snap shot of the entire run? Maybe you have some bad sheets.
Also 100m / 100c will produce purple. the chage from "blue" to "purple" usually take place after 80% M with the C being solid, just my 2 cents.
Also 100m / 100c will produce purple. the chage from "blue" to "purple" usually take place after 80% M with the C being solid, just my 2 cents.