It could be hidden characters from the spreadsheet that appear to be new entries. I wouldn't expect the values to sort alphabetically as shown. Have you tried a trim formula, or analyzing the data in the spreadsheet? I had a similar issue to this recently, and could not locate the hidden characters. I had to create a formula with a trim function to remove the hidden characters, and sort/filter off of that field to get the alphabetized list. That field is a unique value in my App, so this issue you show here looks a little different, as Vendor appears to be a text field, possibly a dropdown?
ALLIED
ANIXTER
AVNET
Anixter
Avnet
BISCO
HEILIND
Heilind
Create a new field - Vendor Fx - Trim([Vendor])
Sort by that field and see if the data matches this format
ANIXTER
Anixter
AVNET
Avnet
If so, I am guessing hidden characters, and its a matter of cleaning up data in the spreadsheet before importing. (and a clean up effort in your table via grid edit or export/import)
If not, then maybe the field is case sensitive and the sort order remains a mystery.