In September I travelled from Singapore to the US via NRT, and in November I travelled in the reverse direction from the US to Singapore via NRT. Both trips were operated by Japan Airlines.
In the direction from Singapore to the US, upon arriving at NRT, international transit passengers were made to go through security screening before being mixed into the sterile international departure area of the airport. This agrees with what is stated on Narita's website. However, when travelling from the US to Singapore, immediately upon crossing the aerobridge, international passengers were made to go through the side doors leading directly into the waiting area where departing passengers were waiting, and we simply left by walking out of the waiting area, through the sterile area of the airport, to the gate of our connecting flight. In other words, we were deposited directly into the sterile international departure area, and did not go through any security screening in Japan at all.
Note that the security screening before my flight to the US cannot possibly be a US requirement - Narita only has one single international departure area, and I was free to roam around the entire departure area in both cases.
Why did this happen? Does it depend on the country your flight arrives from? Was there a recent change in policy? I could not find any online sources that indicate that this is now the policy for flights from the US.