I'm aware of large FreeBSD-using companies which are not listed. Some of them go to varying lengths to avoid publicizing their use of FreeBSD, so if it's one of them I could certainly understand why they'd want to avoid having their name appear at the top of the FreeBSD Foundation donors list.
Usually it's a matter of keeping secrets from their competitors. If you know that a device due to be released next year will be running FreeBSD, it could help you figure out what components are going to be in it, for example (probably parts which FreeBSD can already run on), which might tell you something about its performance.