it means unless they tell you differently. for example "you are to be here at 8am sharp, unless stated otherwise". if they state otherwise, it means plans have changed

This sentence is a wonderful example of "legalese"; the particular language used by lawyers, judges, and paralegals. This exact phrase is likely to appear in a contract of one sort or another. Unless stated otherwise is a warning to the consumer that the terms of their employment or involvement may be changed over time. One always has to be careful of someone who claims that their product or service has terms that are possibly changeable.

If the writer means something other than what is written, he/she will state it in the article.