The Federal Reserve study classifies non-ordinary expenses with households by comparing against median data for a region. For example, having a massive house means the square footage is significantly above median for the region and classifies it as a non-ordinary expense. As far as I know, it doesn't look at interest rates on individual mortgages because I don't believe they have that data accessible. It is a pretty sophisticated multi-dimensional approach they look at household income levels, regional economic conditions, household composition, rural vs urban, local cost of living, etc. It accounts for whatever the team of economic phds who designed it could think of with the data they have available.

The point is that the Fed isn't classifying those things. "Ordinary" includes all primary mortgages, whether for a modest home or a luxury pad. It includes all personal car payments, whether for a shared Honda or a Maserati.