How to fix the incorrect sorting of Purchasing documents?

Summary:

When going to manage agreements, our clients like to do a massive search of all of their agreements. Upon doing so, they like to sort the agreement numbers ascending/descending. We have noticed when doing so, the order of the agreements are still not sorted properly. Ex. Agreements listed as: 1, 10, 100, 101, 11, 102, etc. Why is the system behaving this way and how can we fix this issue? Is anyone else facing the same issue? Attaching a screenshot as an example.

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