But in general, quickly explained(as quick as I can). At 4 levels(3,6,9,12) the mod will either add a secondary or upgrade a current secondary. so if its a grey mod and has no secondaries then at each of those 4 levels it will add a new secondary and you cant get more than 5-6 speed in 1 shot. if it is a green mod meaning it has 1 secondary already than 3 of those levels will add a secondary and 1 of them will get an upgrade so if it started with 6 speed and got 1 upgrade of 5 it would end up with 11 speed which is most likely what you had there.
np