Deliberate mistake: Sam, and not the man he leaps into, somehow fathers a daughter with Abigail. Al and Ziggy's answer to "How?" is "We just don't know." Since Sam has always leaped into the physical body of his subjects with no trace of his own physical presence (yes, we see Sam, but no one else does), fathering a child with his own DNA should be impossible. And the characters acknowledging the "mystery" does not excuse the breach of previously established series canon: it merely makes this, at the very least, a deliberate mistake.
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