This is very common. Take a look at the cone and compare it's diameter to the "cone" on the spindle. If the diameters of the inner races is different, and the balls are the same size, then the outer races CANNOT be the same. They may look similar but the ball track will be smaller.

This is obvious if you stop to think about it. Assuming the bearing is radial (it's not, but easier to imagine that way), then the diameter of the inner ball track + twice the ball diameters = the diameter of the outer track. Therefore, if the inner track is smaller, either the balls are bigger, or the outer track is smaller (or both). Then if the tracks are smaller, then they take fewer balls.

BTW - the exact ball count isn't critical, as long is it's not too many. Too few would only be an issue if there was a total gap spanning more than 60° of arc. (minimum number of balls in a bearing is 6, more typically 7 or more are used).