I have found that bubble levels are very often not precisely calibrated. This applies to all sorts of configurations including even some fairly high priced carpenter's levels. I would be surprised if all the inexpensive hotshoe levels available are truly accurate. MANY of the little circular levels I have purchased have not been accurate until I adjusted them (by grinding their bottoms until they are accurate).
Here's a way to test any level's bubbles for accuracy: Place the level on a reasonably "level" surface. Carefully mark the level's position on the surface (with a marker, tape, or whatever). Allow the bubble to settle, and note the precise position of the bubble (the bubble may not be centered, but the surface the level is on may not be truly level). Now, rotate the level on the surface exactly 180° and be sure it is referenced to the alignment marks you made before. The bubble should now be in precisely the same relative position (the same distance from center and the same direction from center relative to YOUR position, not the level's) that it was before. If, for example, the bubble is 1/16" to the right of center (relative to your viewing position) with the level in both positions, the level is accurate.
You can use this method to check a hotshoe level on a camera. If the camera is an SLR, take the lens off so its weight does not make the camera tip. Mount the hotshoe level. Align the camera on a level surface. Check the bubbles' positions. Rotate the camera 180° and check the bubbles again. The bubbles should have the same relative positions to their centers (again, relative to YOU, not to the camera) with the camera in both positions.
All that said, I have never found a need to have a level on a camera. It is EASY to perfectly align a camera to level in the field (for static shots like "landscape") if you have it on a good tripod with a decently calibrated level on the tripod. First, make sure the center column of your tripod is vertical. This can be done with a built-in level or even a quality pocket level. Once you have the camera roughly aimed at the intended subject, make note of an object near the top (or bottom) edge of the viewfinder's frame. Now (with the tripod's center column vertical), pan the camera left and right. The reference object should stay the same distance from the top (or bottom) edge of the viewfinder as you rotate the camera. If it does, the camera is level. If the object-to-viewfinder-edge distance changes, adjust the tripod's head a little and check it again.
Since you usually have a horizon, building (or building part), utility pole, or other naturally horizontal or vertical reference in your viewfinder, you can usually roughly level the camera extremely easily and then use the method above to prove that it's level.