UPDATE: more clarified here:
PS: I have received this piece of information from a source that was reliable in the past. Still, take it with a grain of salt.
A few days ago I received some non-official information, that Wargaming has been stealth-buffing artillery characteristics without making a public announcement to the playerbase.
My contact told me, that artillery underperformed drastically, and tweaks were applied to the tanks or the ammunition (At this point, the contact did not mention precisely what got changed).
The result of those tweaks can be observed on websites that display the gun marks requirements.
Suddenly, after the micropatch was applied 2 days ago, the performance of every arty above tier 3 started to rise significantly (still not to the old level before the massive patch).
The department of Wargaming that studies the live servers (not the Supertest or test servers) had the alarm bells ringing. And has therefore decided to a emergency buff of artillery performance, to counteract player and monetary losses.
The statistics can be seen on sites like tomato.gg that show the gun mark requirements.
I also noticed, that skill4ltu also got a hint of this phenomenon and published a video about this topic on 15.07.2021.