One is 2 ohm..... the other is 4?
...but seriously. The difference is that an amp tends to put out quite a bit more power if it is wired to 2ohm (and is stable at 2ohm). Thus, someone with an amp that is 2 or 1 ohm stable would want to buy a SVC 2ohm sub to get the full power out of the amp.
Then there is the possibility that someone wants 2 subs and has an amp that is only 4ohm stable, then they might want 2 SVC 2ohm subs wired into a 4ohm load for that amp.
It's all about matching a given number of subs to a specific amp.