Each product can only have a single rate assigned to it for a given destination and service. When combining rates for multi-product orders we choose the largest base rate and then use the per product assigned addon rates.
Lets break down example 2, with a slight change to aid readability.
A rate, PCBs Base $2 Addon $0.50
B rate, Others Base $5 Addon $0.75
iii) 3 PCBs $3, only use A rate, $2.00 + $0.50 + $0.50
iv) PCB + other $5.50, use both rates. We choose Base B and then use Addon A. 5.00 + 0.50
iiv) 2 PCBs and 2 Others, $7.00, Use Base B and then the correct addon rates. 5.00 + 0.75 + 0.50 + 0.50
By setting B addon to $0.75 you need to ship up to 6 Other items in a box before you have enough extra shipping to pay for a second box. Larger addon rates would mean fewer items need to be packed together.
Does that help?