Originally, the new round was going to be a 6.5mm, but part way through the testing process they upped it to a 7mm and got bogged down testing a huge number of variations of the basic specifications that got more and more extreme through requirement creep.
If they'd bitten the bullet (excuse the pun) and gone with what they were testing in 1909, the British Army might have fought WW1 with a 150 grain, 7x57mm 2799 fps rimless round, but they were chasing the magic 2800+ fps target and kept on stretching the case out to chase the magic number and started having bore erosion issues.
Hindsight is wonderful.