Tiller is a relative measurement of the preload on the top as compared to the bottom limb. On a true dual cam bow (not a hybrid and not a single cam) the tiller is measured from the limb pocket to the actual bow string, however on a Hybrid or a single cam, there exists the possiblity that the idler and the cam are not the same diameter (like on the Matthews Apex). For those bows, a string is stretched between the axles on the top and bottom cam and tiller is measured from that reference. When you do this, you may notice that the string does not run parallel to the actual bow string.

The axle to axle (ATA) is the measured length between the centerline of the axles.

I can't speak for the Fred Bear bows, but on most single cams, the tiller is rarely equal when measured to the bow string.