My guess is that they gave you the $600 in out of pocket expenses and then a maximum reimbursement of $1,000 for the lost bag. This completely sucks for you, but unfortunately I get it. It comes down to proof. The out of pocket expenses are easy to prove. You show receipts for what you bought. But the value of the bag and contents are near impossible. You could claim that there was a bag of gold coins in there and they would have no way to prove or disprove your story.
Delta has always been super accommodating with me and luggage issues. I've ALWAYS been compensated 100% for expenses I've incurred due to misplaced or misrouted bags. I've never had a bag lost though. I'm sorry the results weren't better for you.