In these machines (anything that isn't in can/bottle etc), they use syrup blocks and dilution to the supposed correct ratios, so wouldn't surprise me if they squeezed it further. Ice probably doesn't help matters either
I just go to the cinema with a bottle of water or juice from home. It's pretty good, got a yearly pass so I go in there near every couple of weeks now.
Yeah... Cineworld rips you off massively as well. Tip for all: Bring your own food/drink. They don't have x-ray machines and metal detectors so just shove it down your trousers
But luckily I have a Mcdonalds and other wonderful food places right next to it. I know which one I'd go for!!