CSN makes errors in calculation.
There is a difference between the real sell in chestshop data and the data stored in csn's database.

Example:
Seller makes a sign:
4 bone for $ 0.10
Someone buys it.
Chesthop tells buyer: You bought 4 Bone from PlayerSeller for $ 0.10
CSN shows to seller: PlayerBuyer bought 4xBone 12s ago (+0.03)

CSN build 54
Chestshop build 341
Paper 1.18.1

Did that change with the latest builds?

Yes csn build 53 works fine
Running build 53 will not fix the old entries, because those wrong values are in the csn database stored.
But new entries will be calculated correctly.