That is an EXCELLENT point. My guess would be that real copies are made on special blank pages with pre-printed consecutive numbers. The numbers are often in red ink and are done in a funny font that the typical word processing program probably lacks.
If it's a forgery, a certificate number can't be used unless the odd font is available. So, the logical thing is to black it out that area as if protecting the individual's privacy. Which as you point out makes no sense; there is no reason to redact the certificate number.
ping