A change of 3dB is normal in attenuation if going from ADSL mode to ADSL2+. Some routers do get the maths a little wrong and display odd numbers sometimes.
What was the sync before?
If running in ADSL2+ mode this is more sensitive to the presence of the ring wire, multiple extensions etc
If you had anything other than the full 8128Kbps sync with the old 33dB attenuation, then you had an underlying issue of one sort or another, which increases the chance of odd behaviour from ADSl2+.
Some people do find running the hardware in ADSL2 mode can produce better results, but with the WBC training mode, too much fiddling can slow you down as it increases the target margins.