Causes of differential N1/N2???

Was taking the old A319 from Gatwick to Orly today when I noticed that the N1/2 readings on the No.2 engine were hard over, but engine No.1 was at about 50%. This was with ATHR engaged and set for 320kts. Switching back to ATHR would bring up the problem, going to manual thrust was OK.
What would likely cause such a situation? I had to take off the ATHR and fly with manual thrust for most of the flight. Should I have landed or was it OK to continue the way I did?

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