A ‘level’ has to have a curve, some have a very slight curve for better accuracy. My Starrette machinist level is the way, very touchy. My 4’ carpenter levels are more lenient. A perfectly flat bubble will never find center. Now to check a ‘level’ lay it down on surface, mark the bubble, then turn 180 degrees to see how close to the mark you made. Split the difference and there’s your perfect spot. The better levels have micro adjustments to set them, they’re delicate so a little bump will make them off. In a hurry, reverse it each time to find its center.