Because the ascii code for a = 97 and the code for b = 98. Look an ascii table and look at a and b. If you want the console to print out the characters 'a' and 'b' then you need to assign them to a variable as a char (not an int).
inside double quotes "a" is a string. Inside single quotes 'a' it is a char. Strings and chars are different in that a string is a bunch of chars (Characters) "Strung" together... It's a different process.