Because one the estimates for both top out out at 375k. Now unless the Xbox one cratered last month, that shows a slowdown in PS4 purchases. If we then consider regardless of the dodgy PR, the Xbox one is picking up, without the PS4 ramping up at a similar pace (which seems unlikely based on what we have seen in amazon rankings) it might show that the PS4 slowing down in pace at one of the most crucial times of the year.
I have no doubt that the gap between the two will not close this year, but if Microsoft have done enough to keep things within 500k by the years end, then that is worrying for Sony in the US.
On a personal level, I do feel of all the SCE branches, SCEA seems the most inept and I wouldnt be surprised of they allow Microsoft to take North america from them in the long run. Worldwide, The Xbox one doesnt have a chance, but in the US, the fight is just starting when honestly, it was Sonys battle to lose.