In fact, for extrinsic hallucinations, even though PTGEN hallucinates less than BERTS2S (63.3% vs. 64.1%), 6.6% of BERTS2S hallucinations were factual, compared to 2.2% of PTGEN.
if we consider factual hallucinations to be valid, this means that even for extrinsic cases, BERTS2S hallucinates the least.