Yeah, one downfall of BA is that you can't export a single batch, only the parent recipe. However, you CAN select that batch and select File >> Promote Batch to Recipe (CMD-R), then select that new recipe and export just that. It's a hack but it works.
As for the off readings you're getting, I dunno. I've used BA for almost two years now, and I haven't seen anything this far off. I enter in recipes from, say, BYO Magazine, and they hit the projected numbers from there to within a couple of points.
Try doing the above steps and post the recipe file. I'll see what I can come up with in the meantime.