As I understand it, Emirates was the first with 10-across in the 777, but with increased seat pitch (35" ?), so the overall seats per square foot remained the same. Other operators then introduced the same width, saying "same as Emirates". but with their existing lesser pitch.
The 777X is having a substantial re-engineering of the cabin walls and insulation to reduce their thickness (this on an aircraft already well known for being the noisiest in cruise). Airlines seem happy now with 10-across, so the only reason they can be doing this, which apparently gives another 6" cabin width, is to shave another inch of all the seats again, as well, and go for 3-5-3 11-across.