Using a command to effect something is too powerful

If you are wanting to effect yourself with lets say swiftness 3, you'll type "/effect @p minecraft:speed 30 3". But if you look left in your inventory, it will show you have "Speed IV". So im not sure if that second number is meaning to add onto an original power of 1, cause if you dont type anything there it will give you "Speed" as in the first powerfulness of speed.