I can't seem to reproduce this. Can you supply more details, e.g. the ranking settings; the version of jforum; your DB server type; and the like?
FWIW, I know there is a minor bug that if you only have special rankings set then people will get the last special ranking. You need to have at least one "normal" ranking with a high min value to keep this from happening.
Also, ranking settings are cached in memory. You may need to force a cache reload via the admin screen to reload changes. [originally posted on jforum.net by monroe]
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