ok, i was following an herblore calculator from here, usually i have no issues.

 

when i bought like 45k unfinished guthix potions and quiet a few silver dust...i went to level an i noticed i wasn't leveling at the time i should have, checked the exp left to lvl an it was half of what was expected.

 

the calculator says 50xp (same as str pot) and i pulled up the xp counter, an made a few pots, i was getting 25xp per potion...i think your calculator is off.

Correct me if I am wrong but don't you need to add garlic to the potion too? That would explain why you would only get half of the xp.

Edit:

Skip my post Sy explained it better then I did :P

Stat Restore + garlic = 25 xp and produces Guthix Balance (unf)

Guthix Balance (unf) + silver dust = Guthix Balance and also gives 25xp

 

You are only getting half the xp as you are doing half the work.