In some circles this is known as the monopoly effect. Just as in the board game, owning a series of the same properties does increase the value of the set, especially as end-user sales possiblities increase significantly.
Typically, an end user may want to own their acronym or name in many extensions, as each extension represents a different marketing possibility. For example, .com is global and .us is specific to the USA. But for a company that had a global presence, including within the US, the ability to reach their audience increases by owning their acronym in these two extensions. What's more, for example, if a company could aquire the .com, .net, .org, .info, .biz, .us and others of the same acronym, they will likely have available to them a multitude of marketing possibilities and can also better insure that they will be found by their client base as they attempt to find their brand.
End user sales drive the reseller marketplace, and as odds of making an end-user sale increase, so do the reseller prices for those names. If names can be matched in various top extensions, the increase in wholesale and end-user sale prices can sometimes double or triple (or more) the otherwise stand-alone wholesale values.
Great topic and there is certainly more to be said about this effect.
zesty