Should Hashrate decrease as more GPUs are added to a Motherboard? Multiple GPUs underperforming

Hello, I am attempting to mine ethereum with 2 PNY Geforce GTX 1070 and 2 Gigabyte Geforce GTX 1070 on 1 motherboard and I am obtaining lower Hashrates than expected.

I have noticed hashrates as follows:
1PNY 25 Mhash/s
2PNY 51 Mhash/s
1PNY 1Gigabyte 51 Mhash/s
1PNY & 2 Gigabyte 61 Mhash/s
2PNY & 1 Gigabyte 61 Mhash/s
2PNY & 2 Gigabyte 73 Mhash/s

I expected each of the cards to give around 25Mh/s and I am suspicious that when more than 3 cards are used the lower rates are produced by the cards for some reason I cannot explain. Can someone explain why the additional cards are not performing as expected or point me in the direction what I should be monitoring like the graphics clock, PCLe link speed, or memory transfer rate?

I have read articles pointing towards overclocking or flashing the GPUs, but I never preformed these tasks and I am unsure if these are the best solutions to my issue.

Also I am using an Intel Celeron G1840 Processor on unbuntu 16.04.

Kind Regards,