Saying that they are the same thing is not a fair statement.
Generally and I say generally as there are always exceptions, a drinking water system uses granular activated carbon instead of solid carbon blocks which are recommended for reef quality systems. In some cases they use a larger micron size and almost always a nominal rated prefilter in place of a higher quality (read: More Expensive) absolute rated smaller micron size filter. They also throw in things like a small post RO carbon filter for taste and odor and count that as a "Stage" of the filtration.
You really have to look closely at what you are buying and read between the lines or you may end up with something that will consume your lunch money budget in replacement DI cartridges or resin. It may also go through membranes faster due to ineffective prefilteration. A granular carbon filter might only adsorb chlorine from 300 gallons of tap water before being exhausted whereas a 0.5 micron Chlorine Guzzler carbon block will conservatively treat 20,000 gallons.
When I say you can use the same filter for both reef and drinking water let me clarify by saying it needs to be a reef quality system to begin with and you jsut use the DI bypass feature for drinking water uses.