For the final of gandha is substituted i when preceded by 1. ut 2. pūti 3. su and 4. surabhi in a bahuvrīhi samāsa ,
Thus the अ of गन्ध is replaced by इ, as उद्गन्धिः, पूतिगन्धिः, सुगन्धिः, सुरभिगन्धिः ॥ Why after these only? See तीव्रगन्धा वातः ॥
Vart:- The word गन्ध is a material noun as गन्धान् पिनष्टि, and an adjective as चन्दनगन्धः 'having the scent of sandal'. The rule of substitution applies when it is used as an attribute. Therefore, not here, शोभना गन्धा अस्य = सुगन्ध आपणिकः \a shop-keeper having many sweet perfumes for sale\.,