It might be the hardware's component's tolerances coming into play rather noticeably, since you have a lot of identical systems set up.
If you don't know what I'm talking about: All electronic components have tolerances that they operate under, since it's very hard to make them perfectly. Take a 68 Ohm resistor with a 5% tolerance. This means that it'll have a resistance of 68 * +-.05 when measured, giving it a range of 64.6 resistance to 71.4 Ohm resistance. What's happening in your systems might be that the processors, GPU, RAM, HDD, mobo have tolerances that they operate under, and they're adding up and being easily found since you have so many samples to look at.
It's not uncommon for identical electrical systems to run differently. That's why you see computer advertised with hardware, rather than benchmarks (eg 3.0GHz process rather than "opens firefox in 1 second").