Due to hurricanes, a portion of the resort is now under water and the maintenance fees are lower for those owners of underwater units. The "sub" refers to the means of transportation used to reach those units.
Seriously, the sub means "subsidized". There were plans early on to build another section to the resort and, thus, share the resort's common maintenance over a larger number of units. Owner's who bought prior to those plans being scrapped are "subsidized" to make up for the failure to reduce the dues by building those other units that were in the original POS.
I still like my other explanation, even though it's not true!!!