It is underneath because it is quiter down there and that's where the action is and they did not want the "bump" on top.
It is not for following terrain, it does some have depression capability.
The "disk shaped bump" is there because you cannot put 10lbs of s#*t in a 9lb bag. The Virginia class has the same sonar arrays squeezed into the bow as the Seawolf class has but due to the fact the seawolf is larger in diameter it just won't fit.
The arrays shown in the picture from the Seneca Lake acoustic test facility point out the problem. The arrays are the passive conformal, passive spherical and the truncated/hemispherical active array on the bottom.
Any explanation passed this point comes under the class of NONE-YA
Hope this clears up all the guessing I often see on the "bump".