Yes the table at your link shows the values for bytes and how they are represented.
Look down several rows to see the value for the character '2'. It shows that the decimal value for '2' is 50.
Look back at the characters you were adding together: '123' they have the values: 49, 50 & 51. Their sum is 150.
Char Octal Dec Hex Description 2 62 50 32 Two
hello sir, oh!, okay sir that's why the sum is 150!...thank you for this idea...