I have been using Capture One Pro for developing the RAW files of my Phase One 25+ back and the results have been excellent.
I took some test shots today which were perfectly exposed on the back's LCD as well as in C1 Pro.
Then I took the RAW files into Adobe Lightroom 4 and also into Photoshop CS 5 (which uses the same camera RAW engine) and to my surprise there was a huge difference, particularly in exposure. Where exposure in C1 was dead-on, it was very much underexposed in Lightroom / CS5 to the point that I needed to increase the exposure tab by some 1.6 stops to get to proper exposure.
I understand that there will be some differences in rendering from one software package to another, but the BIG difference in basic exposure surprised me a lot.
I'd consult with your dealer on such first-time-use questions. It's much faster and requires less back and forth.
But the short answer to this particular question is it's a bug. ACR/LR misread the P25+ raw file and misplace the "0" exposure position. Don't be alarmed; good quality can still be had. Though of course best quality will always be had in Capture One.
Also make sure your P25+ firmware is up to date and if you have especially old firmware (anything older than 484) make sure to consult your dealer on how to do the firmware update - you should not jump from very old firmware to the current firmware.