What problem does this address?

This is a minor issue and functionality is in no way affected. When wp-env is downloading sources, it displays output such as "Downloading WordPress - 50/100%", and the 100 is redundant since 100% is an implied maximum, and "50%" means the same thing and is clearer and more concise.

What is your proposed solution?

Removing the /100 from download-sources.js is all that is needed to resolve this issue. I will be submitting a PR shortly.