I think you're missing a step in there, Mikey, it actually looks more like:
Pale Ale > India Pale Ale > Double/Imperial India Pale Ale
IPAs were developed by the English to ensure their Pale Ales survived the trip from Britania to India during the UK's empire-building days. They did so by jacking up the hop content (and the malt accordingly), as hops act as a preservative in addition to a flavoring/bittering agent. Double/Imperial IPAs are recent manifestations of our hop-crazed "bigger is better" culture. The Imperial designation actually stems from Russian Imperial Stouts, which were stouts brewed for for the Czar of Russia and his Imperial Court during the winter months, and were typically much stronger than everyday stouts. Modern brewers have adopted the term "Imperial" to mean any style where the grain and/or hop bill has been jacked up, resulting in a beer that is markedly stronger (though not always necessarily double the strength) of a regular offering.
Now, how Great Divide takes their Pale Ale and makes it a DIPA is probably a question best left to the brewer, but my guess is they just increased their standard Pale Ale ingredients to the point that the ABV is within the range of DIPAs. They might have been able to do the same by just aging Hercules, but perhaps they didn't like the way the wood aging interacted with that particular recipe.