The Zip Codes data set is an attempt to represent mailing address zip codes as single contiguous polygons. This is not unique to this particular data set. All polygon zip code data sets share this conundrum.
Zip code polygons are used as a geographic aggregation tool. Many types of data are aggregated to zip code including demographic data.
US Census Bureau
MapWise Inc
zip code
Florida
unknown, some adjustments have been made to parcel boundaries
po_name
state
sumblkpop
pop2001
zip
descript
latitude
longitude