Rotor Balancing

I just had a strange one. I have been balancing my rotor system using a DSS
balancer where I would get the ips and phase angle measurements, turn the
rotor to the angle with respect to the photo cell described by the phase
angle information, then sight over the velocimeter and add weight to the
rotor system on the Opposite side of the mast. This Add Opposite has worked
time and time again on the main rotor. Recently I did my annual condition
inspection and went completely thru the controls, re-adjusting every thing
and carefully zeroed out the lead/lag, checked the head shift and then did a
hover check. Results 0.96 ips @ 270. Well with the arrangement I have
with the velocimeter, the photo cell and the reflective tape on the swash
plate this mean that most of the inbalance was chord wise. I added weight
to the side Opposite and flew again. Results: 1.96 ips @ 274!!! I
couldn't believe the datat. For giggles I reversed the weight to an Add
Same condition and got the following results: 0.26 ips @ 184. What
little inbalance that was left appeared to be span-wise. Question: What
could cause the phase angle information to flip 180 degrees from what I was
used to seeing? The only thing that I'm familiar with that causes the 180
flip with all things else being equal is to be operating above a critical
speed and the rotation rate hasn't changed and the mass in the rotor mast
and head hasn't changed. Further I added some washers to the blade
indicated in the 0.26 ips measurement using the Add Opposite technique and
it appears that I should have used the Add Same again. So there seems to be
some strong consistancy in the mysterious 180 degree phase flip.
I expect the Add Same on the tail rotor as I have checked it out and found a
critical speed below the operational speed which will produce the
velocimeter being on the side opposite of the inbalance weight.