Network Speeds?

Hi all, I'm a noob here... Excuse my ignorance... Got a question about read / write speeds across networks.

I have 3 systems connected via Gigabit ethernet switch. Testing Read / Write via Blackmagic Disk Speed Test.

When I test from machine 1 (Mac Pro) to Machine 2 (iMac with LaCie Thunderbolt Drive) I am getting:

Write = 81mb

Read = 68mb

When I test from machine 1 to Machine 3 (Old Mac Pro Cheesegrater - Internal WD Red) I am getting:

Write = 109mb

Read = 109mb

Any one know why there would be such a difference?

The reason I ask, is i'm looking at installing a shared NAS between these three systems, what's the best way to test required network speeds on a Mac, beyond what I'm doing with the Blackmagic app? I've concurrently streamed sessions around 20-40gb from one side to another at the same time fairly well, but trying to educate myself more before I take the leap. Post production sound if that's relevant.