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A warm welcome to the Fitbit Community @Abisland! 
Nice to see you here, in fact that's a really good question. We need to understand first how your Fitbit Zip is calculating your calories. Fitbit account provides an estimated amount of calories burned taking in count your BMR ( age, gender, height and weight), so even if you're not wearing your Fitbit or if you're in a sedentary state your Fitbit will calculate your calories your body has burned during the day.
The total amount of calories that Fitbit provides takes into account your BMR, plus the activity recorded by your tracker, and any other manually log activity. As you mentioned using a HR tracker gives you a better accuracy since it monitors your heart rate while tracking activities from your Fitbit. But Zip accuracy is really good and the difference shouldn't be that many. We've received really good feedback about it.
If you check your Fitbit.com Dashboard under the section Log > Activity > Calories; you will see more details about it. For example, this graph is showing the time line of my calories for this day, as you can see from 12:00 AM to 11:59 PM there is an specific amount of calories calculated throughout the day (the tiny red lines), also from 6:00 AM to 6:00 PM you will see the large lines that shows calories burned by exercising.
Hope this helps, I'll be around in case you have more questions! 
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