There was a survey done, in which an alarmingly high proportion said that they thought that "falling inflation" meant that prices were reducing.
I suspect that Sunak is fully aware of how stupid the masses really are, and was intending to deceive in promising this, happy to appear to be achieving more than was actually being done.
We've had similar crap with them promising to "reduce the deficit" before. Which means that they're reducing the additional amount the nation borrows each month. But we're still borrowing more, and the total debt is increasing. Many think that "reducing the deficit" means we're paying down our debts, one or two politicians even made speeches in which it was obvious that they thought this too. In fact we're continually borrowing more and more, sinking further into dept but at a slightly reduced rate.
A reduction in the rate of increase is still an increase.