When the farmer finishes tithing his produce for the year, the Torah tells him to disperse the tithes to the Levi’im (Levites), orphans, widows, and poor so that they may claim it. We then declare that anything designated for others has been divested/given, that we did not stray from any commandments, and that we have not forgotten (26:13). Why would we have to declare that we followed all the commandments AND that we did not forget? Wouldn’t one imply the other?
Oznaim LaTorah suggests that the commandment to not forget references a previous commandment to not forget what Amalek did to us. While Amalek attacked our weak, we are commanded to give extra attention to those less prosperous and privileged and help them as best as we can. Following commandments is an attestation of our personal adherence to the Torah’s laws, while declaring that we have not forgotten is about helping others achieve their own personal growth.