I think we'll find out soon who's underestimating and who's overestimating what US is producing. However, all the official sources from US disagree with you right now. And again, Ukraine isn't a core interest for US and they're not going to send every last bit of weaponry to Ukraine. Here's what Obama had to say in 2016:

Obama declares Ukraine to be not a core American interest and that he is reluctant to intervene in the country, because Russia will always be able to maintain escalatory dominance there. “The fact is that Ukraine, which is a non-NATO country, is going to be vulnerable to military domination by Russia no matter what we do.”

Meanwhile, Russia was producing over a million shells a year even before the war started, and has only increased up production since then

• https://jamestown.org/program/russia-struggles-to-maintain-munition-stocks-part-one/
• https://jamestown.org/program/russia-struggles-to-maintain-munition-stocks-part-two/

CSIS just published an article admitting that Russia isn't going to be running out of missiles either https://www.csis.org/analysis/russia-isnt-going-run-out-missiles

tank production in Russia is ramping up too https://bulgarianmilitary.com/2023/06/29/russias-uvz-halts-all-secondary-output-begins-tank-mass-production/

The reality of the situation is that Russia has a state run military industry that it inherited from USSR, while the west has privatized industry. It's much easier for Russia to ramp up production than it is for the west.

Finally, there's China with an industrial base that dwarfs the west. China needs Russia to secure resources and as a shield in the west. The worst possible outcome for China would be if Russia was balkanized by the west and US was able to install puppet regimes there. If Russia ever found itself in a difficult position then China will step in and provide support.