The parties do update according to their available evidence. However, neither has access to all the evidence. Also, evidence can be misleading - and subsets of the evidence are more likely to mislead.

Parties can become less accurate after updating, I think.

For example, consider A in this example.

For another example, say A privately sees 5 heads, and A's identical twin, B privately sees 7 tails - and then they Auman agree on the issue of whether the coin is fair. A will come out with more confidence in thinking that the coin is biased. If the coin is actually fair, A will have become more wrong.

If A and B had shared all their evidence - instead of going through an Auman agreement exchange - A would have realised that the coin was probably fair - thereby becoming less wrong.