I'm willing to bet that their reports are entirely based on annual reports submitted by the telecom industry. The FCC probably doesn't have the manpower to do the surveying on it's own (or the funding to create such a task force), so they just ask ISPs to tell them what services are offered and where on an annual basis.

Unfortunately, they probably also lacks the funding to provide an auditor for those reports, and I bet there's no regulation requiring the industry to have an FCC auditor for them.

In other words, Congress doesn't care how accurate the report actually is.