ahh, it's easy to do. Let's take your example of going east. Get on the top of the sector you're going through, and gradually drift into the sector above, while still going mostly east. On BAR it looks like your flight is heading to the North, while in reality you're heading East. This can really throw off an intercept because you don't go directly to the sector, you go ahead of where you think the bar is going. So to us, it looked like you were going North, so from where we were, we started heading Northwest to intercept you heading "North." In reality the buffs were heading East at this point, which we didn't realize until the BAR went to the sector down and to the right (we were expecting up, up to the right or just right.) It was about that time I said "umm, they might be going to Malta." We made best speed to Malta, and the B-17s had already dropped and were turning to make a second pass.
So what probably happened is you unintentionally did what I said. Skirted a sector line and made it look like you were going North when you were actually going East.