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It occurs because a Presence Map (or PMap) is considered to have an infinite suffix of zeroes.

If the PMap has no more bits set to 1, it is truncated to its shortest form (number of bytes required to represent all bits set to 1 without trailing bytes with all zero bits). Any field that requires a PMap bit from beyond the end of the PMap bytes must infer a 0 bit.

In the code implementing a PMap representation, it is simple to return a 0 bit if there are no more actual bits in the PMap store.