The added vitamin C Ascorbic acid is the form of vitamin C that causes a significant reduction in potency. The vitamin C in Lemons and Lime I agree is probably the reason citric acid solution seems to work better. Other impurities also probably get in the way of the citric acid in the Lemons/Limes.
Lemons and Limes taste better though and have a health effect of their own. Bioflavonoids and an alkalizing effect come to mind.