It could also be this: because you're in North America, where the Euro hams are aiming their beams, you are in the target area of their signals, so naturally you may hear them.
But because the US and Canadian hams are aiming their beams at Europe (and possibly away from you, if they're in the NE or Mid Atlantic US, or Atlantic Canada) you can't hear them because you're in back of their beam antenna, or off the side of their beam where the signal is reduced.
The signals may also be skipping over you, as Chris S said.
I'm here in the NW US and half the time I hear Europe on the ham bands I don't hear the US or Canadian stations talking back at them.