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Because in between Z2 and PCB1-1, you would not even fit nut. So there are just 3-4 washers. With PCB2-1, there may be a bit more space but still it would be quite hard to manipulate the nut.
On the other side between Z1 and PCBX-2 there can be two nuts and washers instead of so many washers, but I found it easier for tightening when setting the correct distance between Z1 and Z2.
Anyway. Anybody can do it as he likes.