Searching / Indexing Issues

We have a Metadata field Group which stores only alphabets (A-Z).

While doing testing , I found some strange issues:

In the search profile, when we enter the group field values with alphabets like X,M  , the results returned are correct,

whereas if I input the value as A, T ,S it returns the whole set of contents ( as if completely oblivious of the input) and these contents dont even have A as the field value.

Quite amused at this. Any help would be appreciated.

Thanks in anticipation!