I'm sure that's not right - any individual can request a remark (or various versions of it - admin check, arithmetic check, full remark). For an individual remark, makrs can go down as well as up but most often stay exactly the same.
For a group remark, marks cannot go down - only stay the same or go up.
You should take advice from the school - they have all the details of marks for different modules. You will be able to see if one module was subject ot mdoeraiton, for instance, and whether it applied just to your DD's work or across the year.