Azure Devops branches commits

I have a 'main' branch and one 'dev' branch. After I create a PR and merge dev into main, the branches tab shows '1 Behind' and '1 Ahead', and for each merge one number increments on both 'Behind' and 'Ahead' in dev branch. But the content from files remains the same from both the branches. These numbers are misleading, It shouldn't be same like '0 Behind' and '0 Ahead'? Or Is it necessary to always delete the 'dev' branch after each successful merge. Please help me to understand this.