It's happening gradually. The difference increases, then decreases, sometimes it goes the other way - the calculated becomes greater then the indicated. In general, if there is some kind of pattern, I can not get it.
Most of the observations were done on trans-Atlantic (E-W and W-E direction) routes.
Do you think it could have something to do the sun heating the wings unequally?
And, BTW, I've never set foot in Singapore, unfortunately.
It looks like the matter has been discussed in a lot of 777 cockpits...
Good flying to all!