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I am reading Andreas M. Antonopoulos's book and In chapter 10 - Assembling and Selecting Chains of Block he writes:

Under most circumstances this is also the chain [main chain] with the most block in it, unless there are two equal-length chains and one has more Proof-of-Work.

Can somebody explain how this is possible? How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets? Not sure how this could happen, but this is my only hypothesis.

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How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets?

That is correct. The bitcoin protocol actually follows the chain with the highest accumulated proof-of-work, not the largest block height.

I believe the original implementation followed the largest block height, but that left the network open to certain attacks, so it was soon changed to largest accumulated proof of work.

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