The Eibach springs often lower by different amounts to what is on the box, as the same springs are designed to fit a few different engine configurations
I think the Spider has a bit more weight over the back than the Brera 2.4, so it will be lower in comparison.
It's the same with the OEM springs, certain engine/drivetrain configurations sit higher/lower than others; EG the 2.2 Brera is noticably lower than the 3.2
If the rear of yours had gone down less than expected I'd suggest that they'd sent you the V6 Q4 springs, but the rear being lower doesn't imply that the springs are wrong.
When I put Eibachs on my 159 TBi initially the front was only 10mm lower and the rear 15mm, and after a while they settled to 18mm and 25mm - still quite different to the claimed 35mm ; but to be honest I didn't want it that low so was happy with the result