DMA mode selection question

Hi all,
I've got an older i386 (PI) with an IDE drive that supports Ultra33, and
for which NetBSD (3.1) aggrees yet sets the drive to the next mode down:

# dmesg | grep wd0
wd0 at atabus0 drive 0: <FUJITSU MPB3043ATU>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 4125 MB, 8940 cyl, 15 head, 63 sec, 512 bytes/sect x 8448300 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2 (Ultra/33)
wd0(piixide0:0:0): using PIO mode 4, DMA mode 2 (using DMA)
boot device: wd0
root on wd0a dumps on wd0b

The IDE controller is an on-board Intel 82371SB IDE Interface (PIIX3)
(rev. 0x00) and the IDE cable is a std 40-pin/40-conductor which should
be fine up to Ultra33 speeds.  The kernel is just a paired down GENERIC
with the wd(4) flag set to 0x0000 ( uses whatever mode the drive says it
supports). Any idea why the drivve isn't getting the highest supported
setting?

Cheers,
Jeff