It's tiered per % of ABV so between the 2.8% and 7.5% bracket the rate is 19.08p per % per litre they produce, so a 4% beer pays 19.08p more than a 3.9%I can’t think of anything on draft other than mild, that’s below 3% ?
Stella lowered their % about a decade ago so that it sits just below the 5% mark, hence saving them a fair wack on the duty they pay and pass on.