Using /effect with the amplifier specified gives an effect with higher than expected strength

If you use the /effect command like so:

/effect <player> 1 10 1

The player gets Speed II for 10 seconds NOT speed I, and if I had said 1 10 2 it would be speed III, and 3 is IV. However 4 is also IV, but dose not show the power level in the inventory screen

I would have expected that 1 = I, 2 = II, 3 = III, etc. However the amplifier adds the number to the base of 1, instead of setting the strength as expected.