But for a better explanation:

https://www.bbc.co.uk/news/uk-41059610.amp

Tax is lower on 3.5% and below drinks. The ABV on a label is allowed to be out by 0.5%, so your 4% labelled beers are actually being brewed at 3.5% to lower the tax burden.

There's also genuinely an increasing market for lower strength beers too.