The actual cost of manufacturing a soft drink is in the order of pennies - [0] does a breakdown where the numbers are likely wrong but in the right ballpark. An increase of a few c per litre in tax at the production level would have a huge knock on effect on the price of the drinks, unless the manufacturers eat the increases.
Customers are also hugely price sensitive, an increase of a few pence can have a massive change on purchasing patterns.
[0] https://www.quora.com/How-much-does-it-cost-to-manufacture-3...