As others have said, this is normal. It can also happen as a result of aircraft configuration changes. An example would be my upcoming flight to ANU from LGW which swapped to a LHR configured 777. The WTP cabin has gone from 52 down to 40, CW from 32 to 48. The current price for WTP on that date is £2400 ( W fare class), whereas CW is £1600 ( R fare class).