SDIFF shows the items that are in the first set but are not in the succeeding sets. In example below, sets visitors and visitors6 only has one difference: guest6 in the latter. The first SDIFF command shows empty result because all the items in the visitors set are already in visitors2, but the second command shows guest6 because the command compares visitors2 against the other sets.
127.0.0.1:6379> SMEMBERS visitors
1) "guest1"
2) "guest4"
3) "guest5"
127.0.0.1:6379> SMEMBERS visitors2
1) "guest1"
2) "guest4"
3) "guest5"
4) "guest6"
127.0.0.1:6379> SDIFF visitors visitors2
(empty array)
127.0.0.1:6379> SDIFF visitors2 visitors
1) "guest6"
127.0.0.1:6379>