The RAW files in Bridge may have no precessing applied, which is why most RAW files look dull and flat. Whereas when you open the files up in in ACR, you may have some processing automatically applying, either a Camera Calibration has been set as default, and/or you have set some default preset settings. I have a certain amount of sharpening, noise reduction and lens correction as my default when a file opens in ACR for example. Oh, and the Adobe Standard Camera Calibration.
Once you have opened a file, and then clicked 'Done', when you go back to Bridge, the thumbnail should update if you have changed something or had some default settings applied. If you click 'Cancel' in ACR, no processing will be applied.