Food Inflation in the UK: Why is an incorrect figure being quoted?

I talked about the increase in the price of food a few weeks ago. I found that the price of food for the products I buy if I were to continue buying those precise same products had roughly increased by 15% from July 2021 to July 2022.  

I was astonished to read a few days ago this article claiming that food prices have only increased by 10.6% in the past year.  Astonished, since the price of food has only been accelerating in the 2 months since I wrote that other blog post in late July.  So it seemed to me that the price inflation of food would be even higher than it was in July i.e higher even than 15%. So I decided to check the prices out by comparing some of the food prices I bought back in September and October 2021 from Morrisons and Sainsburys, and comparing to what the current prices are now on the 30th September 2022.  In the below table I’ve mainly included the more common foods that many people frequently buy e.g. bread, potatoes, milk, cheese etc.

Obviously, a lot of food cycles up and down in price all the time, with the low end of the cycle being labelled "offers". I always use prices at the low end of the cycle.

Weight (kg) Product 2021 2022 % Increase 4 pints Fresh Milk £1.09 £1.55 42% 0.4 Cheddar Cheese £2.00 £2.75 38% 0.2 Pecans £2.50 £2.83 13% 0.2 Walnuts £2.00 £2.22 11% 0.4 Warbs Wholemeal Bread £0.85 £0.90 6% 0.2 Morrisons Plain Yogurt £0.50 £0.75 50% 0.15 Walkers Crisps 6x25g £1.25 £1.50 20% 0.4 Jacob's Cream Crackers 2x200g £1.60 £1.80 13% 0.095 Nescafé Gold Blend Espresso Instant Coffee £3.00 £3.50 17% 850ml Ribena Squash £1.50 £1.50 0% 1 Fresh Chicken Drumsticks £1.35 £2.25 67% 1.35 Fresh Whole Chicken £3.00 £3.50 17% 0.3 Morrisons 10 Haddock Fish Fingers £1.75 £1.99 14% 0.5 Morrisons Chunky Battered Haddock £2.99 £5.25 76% 0.4 Bassetts Jelly Babies Sweets Carton £2.00 £2.00 0% 0.2 Morrisons Fruit Jellies £0.90 £1.25 39% 2.5 King Edward Potatoes £1.50 £1.75 17% 2.5 Morrisons Baking Potatoes £1.25 £1.85 48% All Products £31.03 £39.14 26%
There are 18 food products, and as an average they've increased by 26% (highlighted in yellow). Out of the 18 food products listed, only 3 of them have increased by less than 10.6%. It seems to me to be unrealistic to see how that figure could be dragged down to 10.6% by considering more foods. So how on earth is this 10.6% figure being derived? Incidentally, note how much cheaper the haddock fish fingers are to the chunky battered haddock.  And yet they have a very similar percentage of haddock fillet per unit weight (64% and 65% respectively). Fish fingers are significantly the cheapest way to eat fish, more so if one just buys pollock fish fingers (they don't label them as pollock, of course, since they want customers to assume they're cod.  They use names like omega 3 fish fillet etc, even though the omega 3 content is only a small fraction of fish like mackerel and salmon). Original screenshot of spreadsheet below (I couldn't put it in the middle without messing up all the formatting).