In your build instructions for Node apps (Manage build dependencies | Netlify Docs), the recommended way to set a Node version is via a .node-version file or NODE_VERSION env var; and similarly use NPM_VERSION to set the npm version, and YARN_VERSION to set the yarn version.
The approach you’re using can lead to a situation where the Node, npm and/or yarn versions used on the developer’s machine are not the same as in a Netlify build, leading to build failures or strange variation.
Don’t Repeat Yourself – this info should be in only one place. It would be better to use the package.json mechanism. If that can’t be done, it would make sense to add a prominent warning to your doc page that I referenced about to say the standard Node approach is IGNORED.
What site is it? Chances are you started building on Node 10 at first and we don’t upgrade the Node Versions automatically to prevent errors due to breaking changes. All new sites use Node 16 by default, which will likely be pinned too, so they don’t break in future.