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 ****ft and then did
a 
hover check.  Results    0.96 ips @[EMAIL PROTECTED]
 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 @[EMAIL PROTECTED]
 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 @[EMAIL PROTECTED]
 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.