Horizontal cuts are evil. They are the number one 'bottle bouncer' in my experience. It can be very frustrating regardless of whether or not you're attempting to cut a single bottle or if you're attempting the more difficult returning cut doubles. Don't get dishearted though, practice makes perfect but if you're looking for a few pointers then here we go.
'Horizontal' doesn't have to be dead horizontal. When you're trying to cut the bottle I'm pretty sure that in your head youre imagining a perfect line from left to right sitting at exactly 0 degrees. This doesnt have to be the case. I have found that by angling the cut downwards slightly you have less chance of smacking that bottle across your garden. It doesn't have to be much but the speed of the cut and the slight downwards angle together, seem to have the effect of pushing the target down onto the post a little and allowing your blade to bite through the first layer of plastic. After that it's pretty much plain sailing.
Use your body in the cut. With diagonals its easy to use your body because it feels natural. With horizontals it doesnt feel so natural and people can end up hardly moving at all. As you accelerate the tip towards the target, engage your core.
With returning horizontal cuts, it's important to try and only cut as far as you need to because it will make bringing the blade back and around the target that much easier. Lining up on the second bottle to repeat this cut is tricky, but on the second swing you can put more 'oomph' into it to ensure the cut takes as you don't need to bring it back again for another bottle... unless you're doing triple horizontals of course and in which case I wish you the best of luck. :p