My security clearance just granted but the stated reinvestigation period very strange

My Security clearance intiated in June 2015. It was completed on August 2016. Adjudicated and granted on the week June 2019. It stated on my granting notifications that my clearance will be re investigated on or before 2026. But I thought it should be 2029( 10 years after it’s granted). I can’t understand.

10 years from the start of the previous investigation.

Its 10 years after the investigation closes from my understanding, so that timeline matches.

10 years from August 2016 the investigation was completed

Your understanding is correct