The accuracy of the calculator is completely dependent on your ability to forecast your requirements and usage—not an easy thing to do when you first start using AWS!
Unfortunately, not all AWS services are present in the calculator (a notable omission is AWS Lambda). For those services, you will have to do your own calculations based on the service-specific pricing pages.
Some services (for example, DynamoDB, Lambda, and so on) have a free tier that applies even if your account does not qualify for the standard free tier (that is, the account is more than 12 months old). These services will have a note on their specific calculator page, detailing the inclusions:
Keep in mind the expiration date of the free tier when calculating your total annual costs.