Dell Precision T3500 - DDR3-10600 seen as DDR3-8500?

Hi there.  I'm a PC refurbisher, we do a lot of Dell hardware here.  I'm having a bit of a mystery here with the Dell Precision T3500 and it's memory configuration.

I've consulted the memory configuration guide, and came away with the following facts:  T3500 supports upto 24GB RAM total, ECC or non-ECC, 8500 (1033mhz) or 10600 (1333mhz) DIMMs.

I presently have been configuring T3500's with six 2GB DIMMs, 10600U.  But when I look in the BIOS setup, it sees them as 8500's.  I also ran memtest86+, which reports the SPD of all six DIMMs as DDR3-1333 (10600), but runs tests at 1033mhz as if they were DDR3-1033 (8500)

Suspecting BIOS might be the cause, I updated to A17, but no change in memory configuration.  So what gives?  Does it not support 10600U's?  Perhaps one has to use ECC 10600's to get that speed?  It's not exactly clear why it's running these DIMMs at the slower speed.

If it matters, the T3500's I've been working with are using a Xeon W3565 3.2ghz CPU.  Any insights into why it's not working as one would expect would be great!