It seems like all cycling water bottle caps are designed such that you cannot drink all the liquid. Something is always left inside. Is there any purpose in this? I only saw one water bottle that had a cap that allowed you to completely empty the content while drinking. But it came as a free gift with some Dextro Energy drink.
If by design, maybe to keep you from an unexpected swig of gritty, undissolved mix. That’s all I’ve got.
I think it’s mostly just the flattish nature of most lids. This means that, unless you’ve got the bottle completely vertical (upside-down), you’re always going to have a bit of the contents caught in the shoulder, below the valve.
Why are the lids always flat? Well, I’d guess a combination of (a) ease of manufacture, (b) structural considerations, and (c) avoiding adding extra height to the bottle.
The other element that plays into it is that the valve often has a retaining structural element that stands proud of the lid, which is another barrier that the liquid has to get over. It usually has channels in it, but unless you happen to align the channel with the most downward part of the cap, it’s still going to cause some obstruction.
I don’t think it’s an intentional conspiracy to steal your water, though. 
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OP, sounds like this calls for a class action lawsuit!