Upper and lower Characters in Excel

[CamiCat]

Hello friends,
I have to read a row item corresponding to a particular column
I have created a dictionary to match the key=value of 1° column with the values=number of days in the 2° column.

When I try to read the key value as a string I get the following

So the uppercase characters in the Excel FIle have been switched to lower case: how is it possible?
how can I maintain the part in uppercase the same as in the Excel FIle?
Thank you so much friends for your precious help,
Camilla