Many statutes include lists of substances that are subject to that law. These lists generally remain static, changing only through new Congressional action, such as an amendment to the law, or through the regulations that promulgate the law.
The list of substances in a system that supports a statute, however, may be a subset of the list of substances in the statute, or there may be additionally reported substances. These additional substances may be subject to the statute through a new regulation that expands on the statutory list. Another reason is that states, tribes, or industrial facilities may report substances that are not subject to the statute or that are ancillary subject to the statute. In other words, the list of substances in a system is much more likely to change from year to year while the list in a statute reflects a document enacted by Congress at a single point in time.