What was the full list of ingredients used for each potion?
The recipe combinations are somewhat complex, so it wouldn't surprise me if this was simply a side effect of the way the game calculates combination effects as the order they're added seems to make a difference to the outcome.
Lady's Smock doesn;t count as a health potion, so effectively you're only getting the duration for a two ingredient potion. BUT you do gain the primary effect of Lady's Smock which is Major Prophecy (spell critical damage). That same page has a bunch of potions you can make to gain a secondary affect while seeing the duration reduced from 52.8 to 47.6 s.
There's also a other combo with dragon thorn. In other words, you need columbine and either lady smock or bugloss... and mounta9n flower or dragon thorn... something like that
If all 3 ingredients share an effect, it does not increase the power of potion but does slightly increase duration compared to 2 ingredients with effect.