Not sure where you are looking, but the markings on those radiation patterns are 10 at the edge of the radial, then 0, -10 and -20 moving back to the centre of the diagram. On the bottom left diagram the line goes right through the middle of the 0, so in that direction (90 degrees) the antenna gain is 0dB. In the middle diagram the gain at 90 degrees is about -1dB (or thereabouts).
Your statement is correct regarding the signal being stronger above it, than directly in front of it if it was suspended in space in the same orientation as if it was wall mounted, but 1dB isn't going to make a huge difference.