There are several differences, but here's one. The LH-55 and Sirui are both manufactured for the photo market. They have significantly more parts for fine tune control. All those parts need tolerances which have to be "taken up" when the ball lock is tightened. This can actually be seen through the scope as POA shift.
The Anvil has fewer parts, tighter tolerances, and there is solid contact between the rifle, head, and ball during operation, and I've found it results in essentially no POA shift.