Credit balances can occur if you modify your subscription with a change that pro-rates a credit.
For example: In the middle of my billing cycle, you reduce the number of devices from 10 to 5. This will give your account a credit for half of a month charge for 5 of the devices. This pro-rated credit can then be applied to your next bill.
However, if you cancel your plan prior to the next bill, your account is put in a cancelled status on your next billing date. These credits will no longer be available to you upon that cancellation.
If you then re-activate your account after the cancellation has gone through, you will see a "balance reset" on your bill. Re-activation sets your account balance to zero, so this transaction will remove the credit from your account.