The optical effect you describe comes in part from the shape, but also because Walsh Walsh / Arculus and Richardson used high lead content glass - around 32-33%. That gives it a higher refractive index than 24% lead glass.
A slight nipple or 'cow lick' on top is not uncommon on OE weights (and other antiques), and arises from the blocking process at manufacture when the dome is shaped. Occasionally you get a flat spot at the centre. You often have to look hard to spot these defects though, which sometimes distort the very centre of the design. Many antique weights have been repolished of course, and so the original blocking marks and any defects tend to have been removed.