Translation:I go to the library at least once a week.
Because you're putting a minimum on an amount, guaranteeing that more is taken as standard. Think of it like putting a minimum price on an item you're selling. By doing that, when an offer comes, it is for more. You always meant for more, and when you say it in English, you meant for it to be thought of as more.
Yes, of course it can be the the stated minimum amount or more - that's what "at least" means, but the point is that when something is "at least" something, it's generally accepted that what it is, is more than the stated minimum amount - hence the "standard" I chose to use.