I know the OP said they already got their answer, but let me explain (and I believe this is already mentioned in the EVs/IVs thread)--
The IV calculator is ONLY 100% accurate if your Pokemon is at LV 100 (with no EVs, obviously). Before then, there is a margin of error. So if the calculator gets more than one value for an IV (for example, Speed could be anywhere from 20-24) it will use the average of those values to determine the HP. Since the averages are just an estimate, you won't get an accurate HP (or power of your HP) unless the calculator can determine accurate IVs.
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-Answered, closed