Why the total looks different
VA uses a combined ratings table to account for multiple service-connected disabilities. Each additional rating applies to the remaining portion of the whole person, rather than adding directly to the previous percentage.
A simple example
If your only ratings are 50% and 30%, they combine to 65%, which rounds to a 70% combined rating when no special rules apply. The result is not 80%.
Check the full picture
VA rounds the final combined value to the nearest 10%. Other ratings and special rules can affect the calculation. Review each individual rating and your combined rating in the decision letter before estimating a change.