T3500 -- difference between Slot 1 and Slot 3 (both x4)

I have a T3500 that currently has 2 additional SATA adapters in it due to some testing.
1. SYBA SD-PEX40055 with Marvel 88SE9130
2. SYBA SD-PEX50063 with ASMedia 1061
Both adapters are SATA 3.

I discovered that when the PEX40055 is in Slot 3, it is capable of a sustained DTR of about 120,000 kb/s in the erase test with a 3TB Seagate.    However, when the same card is in Slot 1, the it is capable of about 270,000 kb/s.

To make sure I'm not just imagining things, I repeated the test with the other card using the ASM 1061.  There was a tiny bit of difference:  Slot 1: 350,000 kb/s; Slot 3: 325,000 kb/s.

For reference, the onboard SATA 2 controller had a DTR of about 245,000 kb/s.

Does anyone have any idea why this is happening?  

Originally, I thought Slot 3 may have less lanes, but the test with the ASM1061 based card showed that is not the case, but it was still slightly slower.

I don't attach all the test results [unless someone asks for them], but here are the results with Marvel based card that showed the more than 2x difference.

In slot 1:

In Slot 3: