There are two version of the 3-Class 777s, one has rows 1-4 in the forward cabin, and the other rows 1-5. I believe the seat plan initially shows the seating map for the aircraft with rows 1-4 and then closer to departure updates to the actual aircraft, which may have the extra row 5.

It's also possible the opposite will happen, you select a seat in row 5 and then at the last minute the aircraft changes to the other variant without that row....