Global text search does not show results sorted in alphabetical order, this is by design.
Search results are sorted based on the retrieved individual records scoring, calculated on a sum of three main components. The relevant factors of a document score are: frequency of the search terms, sequence of the search terms, weight of the indexed source fields.
The most relevant high scoring results from a search are always listed on top, following the lower scoring ones in descending order.
More information can be found in the documentation pages below: