In a custom entity, updating specified field(not currency), the field currency also be updated

In my a custom entity, there are many fields, also contain the field currency. The field currency also be updated while I update other field, although the field currency is still original value.

Attached image is Audit History.

Here is a snippet of the code:

Entity vendorResource = new Entity("EntityStaff");
vendorResource.Id = vendors.Entities[0].Id;

vendorResource.Attributes["abc_building"] = new EntityReference("abc_officefloor", buildingID);
vendorResource.Attributes["abc_seat"] = new EntityReference("abc_facilityseat", seatID);

service.Update(vendorResource);

I just want to update Facility Site and Seat fields, but I don't know why the field Currency also be updated. Help me!