This is because seishi are often written by people in service to the dynasty of the time, and after earlier dynasties have come to an end, so there is a tendency to exaggerate the evils of the rulers of the preceding dynasty beyond what is attested in historical records.
理由として、正史とは一つの王朝が滅びた後、次代の王朝に仕える人々が著すため、最後の君主などは実際以上に悪く書かれる傾向にある。