The following letters all have something in common which may not be obvious at a first glance:
A B D H P
No other letters share this attribute.
There are no misspellings or typos in the title of this question. Maybe a clue though.
More hints may follow if the question is not answered.
Some great answers so far all of which I have upvoted but none of which are exactly what I am looking for.
0and the character
(also have the same property. I only said no other letters share it ;-)
As correctly identified by @RedBaron the ASCII table is key here. There is a good reason why "sum" is mentioned in the title and there are two reasons why "two" is mentioned.