dna - What is the justification of Chargaff's second parity rule?

First parity rule

The first rule holds that a double-stranded DNA molecule globally has percentage base pair equality: %A = %T and %G = %C. The rigorous validation of the rule constitutes the basis of Watson-Crick pairs in the DNA double helix model.

Second parity rule

The second rule holds that both %A ~ %T and %G ~ %C are valid for each of the two DNA strands. This describes only a global feature of the base composition in a single DNA strand.

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It makes sense that in the context of dsDNA, that A = T and C = G, but I don't see an obvious reason why in a single strand of DNA, A ~ T and C ~ G.