This is a standard hazard of the trade, the same way that restaurant back of house has to deal with sharp knives and fire.

Because humans are lying liars who lie. Humans will lie with spoken words, humans will lie with the written word, and humans even lie in stone carvings. (What, you think the Behistun Inscription is 100% true? If so, I've got a bridge in Minecraft I'd love to sell you.)

Fortunately, there is such a thing as the historical method, the same way as there is a scientific method. Here are some previous threads for you to consider:

• u/mikedash examines the matter of bias here,
• u/Georgy_K_Zhukov does the same here,
• and u/itsallfolklore considers 'objective history'.
• a now-deleted user explains how even deliberately falsified information can be useful;
• u/baliev23 examines how historians determine source validity;
• u/crrpit and u/PartyMoses outline how historians do business;
• there's this Monday Methods post on reading primary sources critically, with multiple contributions from multiple people;
• and a previous time I posted this linkdrop, which also contains further insights as to how historians do business and why some don't even use the term 'bias'.

Another thread I haven't linked before on questions of this type, but is relevant to one of your questions, is u/Iphikrates dealing with the numbers Herodotus gives for the Persian invasion of Greece.