My experience has been that the OLCI offers are often ex-USA (in my case, ex-ATL). But my ex-Europe TATL flights have also had D1 look to be a lot more full than when I was going the other way. So while it could be an availability thing, I suspect it has more to do with other companies having more control of the flight. Even on DL metal, for instance, KLM has a lot more to do with flights leaving out of AMS.

Keith