Odd throughput on SATA SSD's on HP Proliant

I normally run at least one SSD in a VMware server whenever I can.  I like to put the VM swap file and the host swap file on there.  Occasionally a VM or two.  Often times a good RAID card with SAS drives will out benchmark an off the shelf SSD but, we are talking about division of labor here.

My 2nd ML350 G6 for my home lab was kinda pokey...I ran a disk benchmark and found that the SSD wouldn't write beyond 20Mbps.  I assumed I had killed off the Intel 180gb drive.  So I swapped it for a 160gb Intel I had in the parts pile.  The speeds came back, OK great, move on.  Well then later on I freed up a Crucial 256gb drive that was newer, faster, and larger, so swapped it out for the Intel.  The dismal write speeds were back.  I did notice that the ML350 has 6 SATA ports, and VMware reports it as having two SATA controllers, one with 4 connections and another with 2.  I tried different ports, different SATA cables, even different power cables.  Still horrible write speeds.

I gave up and installed a generic SATA 6gbps PCI-e card out of the parts pile that has a Marvelle chip on it.  VMware 6.x saw this controller without issues.  Not only did the speeds come back but they are way faster than a previous test I ran over a year ago, I can only assume that when I ran a benchmark test with this very same controller a year a go, the hard drive in question couldn't fully utilize the 6gbps?

Using onboard SATA controller, on a good ML350.

Using the add-in 6gbps PIC-e SATA controller.

The test I ran with the same card, same SSD a few years ago.

If anyone on knows why the HP had slow onboard SATA speeds let me know, it is kind of a null and void point now, however I'd still like to know.