1. Make a Minecraft world with cheats enabled
2. use the command /effect @s Speed 30 1
3. Check your potion effects

When you do the above steps you gain speed 2 and not speed 1. making it impossible to gain effect that are level 1 example if you did the command but with strength 1 it would give strength 2.  [It works with any potion effect and any potion level.] if you put the amplifier at 0 instead of it giving 1 amplifier it gives none.

It is ment to give speed 1 not speed 2.