Why do interest rates go up for more units? Seems irrelevant.

[Chris May]

I'm trying to buy a 2-4 unit property for 1.0-1.2M. I've been approved by my lender, but at rates of 4.25% for a duplex, 4.375% for a triplex, and 4.5% for 4 units. 

Why does the interest rate go up? I suppose I could understand if I needed the rents to qualify based on my income, but we'll be owner occupying and we're approved for a SFH well above 1.2M.

What difference does it make from a risk perspective? Am I just falling into a one size fits all risk model that doesn't really make sense for my particular situation?

Are there any lenders that will give SFH rates for multi family properties, assuming we don't need the rents to qualify?

It makes the 4 unit properties significantly less attractive from a cash flow perspective.

Hope my question makes sense.