The problem is the child-proof caps. They are a freely rotating cap that has to be pushed down to engage the actual spill proof cap. If there is no strong force between the two the child proof part spins but the sealing part doesn't.

The problem is that they're so cheaply made the same also applies when doing them up. If you don't squeeze the cap sides tightly the child proof bit rotates but the spill proof part doesn't so it doesn't properly seal the bottle. Hence finger tight bottle that would be fine normally is not as tightly sealed against leaks as you (or me, or the factory) think.