The levels used for effects when using the effects code (or using command blocks to create a custom potion) are off by one compared to their non-code names (the names used in the potion names and when the effect is on you). For example the code "/effect @p 19 5 1" gives the nearest player the "Poison II" effect. Not poison 1. Poison 2. To get the regular level poison effect you have to use zero as the level number. This is easy to test by giving yourself the potion, then comparing it to the "equivalent" version using the effect code.

Normally I'd just shrug and say "yet another number system in Minecraft that starts at zero instead of one...business as usual" but it's the fact that the numbers for the same thing disagree depending on where you read them that's the bug. Either the numbering starts at 1 (like it does for potion names and "current effects" reports in the inventory screen) or it starts at zero (like it does in effects commands and the code of custom potions). It needs to be either one or the other in all cases.