Drop Down List allows small and capital letters.

Hi, I have used drop down list and data validation before with no problem. Now I created this data validation drop down list for a column with about 300 rows: A8 ; A28 ; A6 ; A10 ; A8 ; Other - Now what happens is: when I just type, instead of using the selction from the list, I can type with a small letters, such as "a28" instead of "A28" and the entry stays as "a28", it doesn't chage to capital letter, despite the validation was done on capitol letters only. This of course jeopardise the objective of the drop list, with people typing some with a, some with A ending up having 11 different entries instead of a maximum of possible 7. I hope it is not too confusing... remember, the validated date in bold above, is the actual data I need my team to imput (they are code for dialysis fluids, I work as a nurse).
I don't want to end up forcing the entire column to change font to capital letter with the "text" formula applied...
Is this one of the bug in excel?
Is anything to do with the fact that "A28" might resemble a cell in column A at row 28?
Thanks for your help