I don't understand why they chose to make it look different from the movie. It would have been just as easy to make it spot on.
More stable? The original has a tiny base and would be top-heavy.
More capable of holding pressure? The shape could just be better suited to the types of regulations that a food and beverage manufacturer has to abide by. I'm sure when manufacturing bottles they have to be rated ABOVE the max PSI they'd encounter.
More stackable? In manufacturing they often have to be aware of how high they can stack the bottles for storage. With a extremely narrow hourglass shape it could cause the bottles to buckle under the weight of other boxes on top of it.
To fit exactly 16.9 ounces in it? They already have bottling equipment set to fill 16.9 ounce bottles, and in order to not slow their normal bottling line down they could run these at the same time as their normal bottles rather than having to set up machines exclusively for this run.
To fit the standards of their equipment? Similarly, their bottle tracks in the machinery are set to a certain width, I'd bet that if we measured a standard 16.9 ounce Pepsi bottle and this bottle, we'd find they are the same width. Again, preventing them from having to take a full machine offline just to do a limited run.
Limitation of the technology? This is actually the guess I've been tending towards (Along with the previous two). The SU bottle is very slender, and I doubt that once the previous two criteria are met that a traditional blow mold would consistently expand the plastic below the narrow section. So in an attempt to have as little waste as possible and keep the cost affordable, they changed the proportions to better fit the process.
...Take your pick. There are lots of logical reasons why they chose to make them different. Most come down to time and cost. And all of that makes sense for a company who's primary business is not accurate replicas, but selling soda. If they slow their soda productions down, they lose money and we'd never get this run.
-Nick