To Keith's question about differential between reverse directions: my understanding is that most of this is due to weight distribution on the bike. If you're going uphill more of your weight is supported on the back wheel so the front wheel is effectively just a little larger (fewer counts); riding downhill more weight is on the front wheel, reducing its effective diameter (more counts).
If there is a lot of wind you may notice that riding into the wind you get fewer counts than riding with the wind. Same reason, the wind is pushing back on your sail (you) and putting more weight onto the back wheel.
Of course if you have a really strong wind or a really steep hill you get the opposite effect since it's hard to steer straight in those conditions (so more counts).
You can test this out if you want to. Try riding downhill but lean back, try to put no weight on your handlebars. You may be able to see a reduction in this effect-- but I don't think you can eliminate the effect that way as there is still the weight on your pedals and seat, and due to the slope there will still be more weight on the front wheel compared to level riding.