Why is it that the average mortgage rate can be so hard for an actual borrower to get?

Here are some questions and answers.
Q: First of all, what's the average been lately?
A: Average rates on 30-year fixed mortgages reached 4.78 percent the week of April 2, a record low dating back to when Freddie Mac first started its survey of mortgage rates in 1971. That mark was tied when the latest number came out Thursday. Rates began falling in the winter, and slid again after the Federal Reserve said last month it would buy $1.2 trillion in mortgage-backed securities and $300 billion in long-term government debt, which traditionally influences rates on 30-year home loans.

Q: What's the benefit of having a lower mortgage rate?
A: Low rates result in less interest that borrowers have to pay on top of their principal loan balance. Freddie Mac says the low rates have spurred refinancing activity — borrowers who refinanced during this year's first quarter reduced their mortgage payments by about $2.5 billion over the coming year.

Q: So, can I get a mortgage with a 4.78 percent rate?
A: Not necessarily. There are several reasons that borrowers may not get the low rates they expect. First, consumers must realize that Freddie Mac reports average rates, which should not be thought of as a standard, industrywide number. Second, a rate can change several times during the day due to fluctuations in the market — it could be 5.5 percent in the morning and increase to 5.75 percent in the afternoon. Loan rates also vary by type. For instance, Freddie Mac's survey showed Thursday that the average rate on a 15-year fixed-rate mortgage was 4.48 percent this week, lower than the 30-year fixed mortgage. And the size of the loan can affect the interest rate — "jumbo loans," ones taken out for expensive homes, are becoming harder to get and carry higher rates than loans for $729,000 or less, for example.

Q: Let's say all those factors work out in my favor. What else can keep me from getting a low rate?
A: For one thing, mortgage lenders look closely at credit scores. If you have a FICO score — a commonly used credit score — close to 800, you're more likely to get an attractive rate. A borrower with, say, a 650 credit score can't expect an interest rate comparable to the record lows reported by Freddie Mac. Also, it can be hard to get the lowest rates if you want to borrow too large a percentage of your home's value. The cutoff varies — sometimes you can get the best rates by keeping the loan to less than 80 percent of the home's value, but sometimes the threshold can be as low as 60 percent.

Q: What if I manage to snare a mortgage rate in the 4.78 percent range — are there other costs to worry about?
A: There most certainly are. One aspect of mortgages that can confuse borrowers is points, or fees. Points vary by lender: Some are paid at the time of application, others at closing. Higher fees mean more cost to the consumer, and could outweigh the benefit of a relatively low interest rate. Some fees, like title insurance, are negotiable, so don't be shy about trying to get them reduced.

Q: So should I be looking elsewhere, besides these average rates that come out each week, as I try to figure out what sort of mortgage I'll be able to get?
A: Freddie Mac releases its survey of mortgage rates for a given week on Thursdays, so reports about it are backward-looking and not a great way to determine what rate a borrower can get right now. Home buyers or owners seeking to refinance should do their own research. Web sites such as the Mortgage Bankers Association's and Yahoo Finance provide updated rates on all types of loans.

Q: What should I do if I'm looking for a mortgage and I don't have a particularly good credit score?
A: Consumers should shop around and compare rates and fees, despite perceived credit problems. There is no shortage of mortgage brokers out there who want to connect lenders with borrowers.