FMC Entry problem.

Yesterday I did a flight from KMIA to EGLL and found an anomoly when entering the waypoints for the NAT track.All the co-ordinates entered correctly except that when I entered N55W040 the FMC changed it to N54W039.I have just rebooted and restarted and got the same.To replicate:Start the 747 at Seattle.Open FMC.Enter KSEA as Start and EGLL as destination.Goto legs page and enter N55W040 in the legs page. On my unit it changes to N54W039.Also, I can't enter these waypoints on the route page. I note that this is the same with the PSS unit. Is this normal? I assume it is.RegardsRussell Jourdain (ANZ007/ZK-RHJ)