I was mildly infuriated at first but I'm going to assume that this girl just has a high standards and you are a lucky man...
Coca-Cola has known to be a cheap drink sometimes even cheaper than water, but if you've ever had a flat Coke it's absolutely disgusting and no one would buy that.
So if it starts out at 100% when you crack the red plastic cap and will dramatically decrease towards that zero flat disgusting worthless Coca-Cola. She just needs the fresh stuff to make it worthwhile or the rest of us peasants while drink 50% stuff a couple days later.
Still wasteful though, I think they have those really tiny 250 ml bottles which aren't the best value for your money but at least give your fiance the best quality Coca-Cola with the least wasted... But that's if you're worried about wasting carbonated sugar water I suppose.