I am a resident of Japan, and I do some of my investments through a US
broker. I have filed W-8BEN with the broker, and therefore availed a
reduced withholding tax of only 10% for all of the dividend and
interest distributions. However a recent dividend distribution from
"IMPAC Mortgage Holdings, 9.125% Series C Cumul Redeem Preferred
Stock" (Yahoo symbol: IMH-PC), my broker withhold tax at 30% rate.
When asked about 30% withholding rate rather than 10%, they gave me
the following answer:

------ Broker response ------
This is a Real Estate Investment Trust (REIT).  As such any dividends
are treaty as ordinary income and are not eligible for a reduced
treaty rate of withholding.

If we were to pay these at a reduced rate, you would be required under
US tax laws to file a 1040NR form because the income is received from
real property.

Therefore, the 30% withholding is the correct rate to have been
applied to these distributions.
--------------

At the same time, dividends from a REIT mutual fund (Ticker: VGSIX)
withhold tax only at 10%.

My question is, why the submission of W-8BEN form could not be
effective for the reduced withholding rate for IMH-PC's distribution?
What are the other marketable securities those are not applicable for
the reduced withholding tax?

(Additional details on IMH-PC: http://www.quantumonline.com [Quick
search] box, enter: IMH-C)