A statement usually only shows what is due at that time; one cannot expect the statement to show future amounts. You're being a bit too cryptic for anyone to fully answer. Are you possibly talking about a medical bill where the new balance due is the result of insurance paying part of the total charges, leaving an additional amount the responsibility of the patient?
Honestly, without more details, it is impossible for anyone here to provide any answer but "Maybe". Who is "they", and what are they billing you for? What do "they" say the $600 is for?