LTR deceased from 80% to 69% day before closing

Hello, I'm not new to being a landlord but I'm new to private lenders.  I recently have been working with a private lender on an off market duplex estate in a B+ area that a lawyer brought to me.  It has been a long bumpy road including higher rates they originally told, 90 days instead of 45 days, and now I'm told the day before closing I need to bring 31% deposit.   What they told me was, even though I think this place should get $1100-$1200 a month per unit, it was rented before at $900 per unit (well, the dad's son lived in one and he rented the other to his friend for $900) so I can only get a loan to cover 80% of that amount(or something like that).

Does all that make sense?  Is it a sign that this property may not be worth what I think it is worth or is this a common thing?  

The assessment came in at $10k over what I'm paying for it.  The property fits the 1% rule and has some good potential for value add.  I just never had this happen.

I appreciate any advice.