There is huge difference in accuracy between a Russian and a western artillery cannon.

At least in 2022 it was commonly estimated that in order to hit a specific target, you need to shoot about ten times as many artillery rounds with a Russian cannon as you would with a western cannon.

Of course, if you don't have a specific target, the accuracy doesn't really matter. The Russian artillery rounds are very useful for carpet bombing a city, but not for any actual military use. And when you're comparing the efficiency of armed forces, actually only the military uses are relevant. For that use, you can use a multiplier of 10. 25 000 western shells have about the same battlefield impact as 250 000 Russian shells.

Then remember that each time you fire a shell, it causes wear and tear in the cannon. That means, the Russian cannons get ten times as much wear and tear as western cannons. Atop them being of a much lower quality. The Russia uses a lot of shells for obliterating Ukrainian cities. Ukraine does not do anything similar. That skews the numbers even more yet.