EK of course overbooks flights because of no shows. Data taken from EK's TRIPS system for a flight next week shows:

Capacity: 304/42/8
Booked load: 330/44/8
Estimated load: 289/42/8

Therefore the flight is 'overbooked' in economy by 26, but expected to go with spare seats. EK will still gladly sell you a seat for this flight (despite it being overbooked)

Expertflyer therefore correctly (because seats are still being sold) shows '3s' for fare classes Y, E, R, W, M, B

I think therefore that trying to second-guess availability and therefore op-up opportunities is a far from simple process!