I can think of three reasons for that. One of them or any combination of them could result in such an illogical at first glance situation:
• You made an extra payment towards the principal some time ago and now it’s taking effect. The balance has become so low, that even a higher rate cannot push the actual sum of the monthly payment to its previous margin.
• The PMI got cancelled by the lender. Under the provision of the 1999 Federal law, lenders are required to cancel private mortgage insurance on most home mortgage loans made after July 29, 1999 automatically when amortization has reduced the loan balance to 78% of the value of the property at the time the loan was made. An earlier cancellation at 80% of the property’s value is likely to happen only if initiated by the borrower himself.
• If the rate has increased, but the amount of your monthly payment remained unchanged or went up insignificantly and then froze at that level, you may be in trouble, because these are the symptoms of a monthly payment cap in action. Check your mortgage contract: Is your mortgage an adjustable rate mortgage? Does it carry a payment cap? If this is the case, you should immediately look for ways to avoid negative amortization, and fast, before it increases the outstanding balance of your mortgage and wastes a lot of the effort you have put into paying the debt off.

Actually, there is no limit to the size of a down payment. It can even be 0% with certain kinds of mortgages, but you have to be able to qualify for it.  Another tricky part here is the Mortgage Insurance. If your down payment is lower than 20% of the purchase price (or the appraised value, whichever the lowest on the day of purchase) of the property, you will be obliged to pay private mortgage insurance (PMI) premiums until the outstanding balance of your mortgage hits the 80% of the purchase price margin. PMI is usually not cheap, so a lot of people prefer alternatives, such as a conventional second mortgage, a piggyback mortgage (80/20) or LPMI. The size of a second mortgage or a piggyback mortgage is determined by how much cash you are ready to put down, the rest can be borrowed.

If, for example, you are planning to put only 5% of the purchase price down, you can either go for a large 95% first mortgage with PMI, or borrow 80% with a first mortgage without PMI and another 15% with either a conventional second mortgage or a piggyback mortgage. The interest rate on the second mortgage will be higher than that on the first one, but still it will very likely result in lower monthly payments than PMI premiums.

There is also a slight issue of tax deductibility involved. While mortgage interest payments are unquestionably tax deductible, PMI premiums are deductible only for mortgage insurance contracts issued from Jan. 1, 2007, through Dec. 31, 2009.