AWS Docs describe Burst Balance and EEBS IO Balance in the following way:
BurstBalance The percent of General Purpose SSD (gp2) burst-bucket I/O credits available.
EBSIOBalance% The percentage of I/O credits remaining in the burst bucket of your RDS database. This metric is available for basic monitoring only. This metric is different from BurstBalance
However, as far as I know, the docs do not explain how those two metrics are different.