adobe premiere writes:
Enter a value for the level. A negative value decreases the volume level and a positive value increases the volume level. A value of 0.0 represents the original clip’s volume level without adjustment.
http://helpx.adobe.com/premiere-pro/using/adjusting-volume-levels.html
but when a value level is 0.0 in sony vegas it is twice louder than premier's 0.0 volume level with same audio file.