I also came here for this question as mine is dropping, though not so dramatically. Just my 2 cents and I see this post is old:
Typically, taps on the outside of the house are not supplied from water softener treated water. The lines to the bibs come from the water source before it reaches the softener as it's not good for lawn and garden. So you should be able to rule that out.
As mentioned above, Rain will effect as will splash-out (resulting in lower Calcium top off - as mentioned, test your tap). I was very surprised to find that my fill water (from the tap) is only 50ppm CH (and I'm in Austin - I thought we had really hard water). Mine dropped from 140 to 120 in just a couple days due to rain (but that was a couple inches of rain).