You’re right, the wording of the legal description in a mineral deed can make a big difference. Beyond that “county wide” issue, where there is existing production whether the interest a seller conveys ends up being the mineral interest they own in a full tract, or just the royalty they own in a specific existing well, can come down to a few words.
In Texas case law there are also lots of examples of those same type unintended consequences related to land sales where seller’s intented to retain part of their mineral or royalty interest but the simply addition of the word “of” in a deed changed everything.
Before signing those type documents having them reviewed by an attorney who knows oil and gas and is representing your interest is worth the money.