For every aircraft, there are two ranges listed in the game: the smaller one is the max payload range (the point after which you will see weight restrictions), and the bigger one is the max fuel range (at which point you will have 0 payload). Data collectors enter the max fuel range (in statute miles, not nautical miles), and the game makes a rough estimate of the max payload range. As such, the max payload range is not designed to be accurate to payload-range charts. In AE 4, we will use more comprehensive payload/range data and payload calculations, but for now, that's the way it is.
I just verified the (data-collector-entered) max fuel ranges of all 777 models in the game against Boeing specifications and found them to be correct:
The 777-200LR with 3 aux tanks has a max fuel range of about 10450 nautical miles, which is 12083 statute miles. In AE it's 12080 statute miles.
The 777-300ER has a max fuel range of just under 8500nm, which is 9782mi. In AE it's 9825mi.
The 777-200 has a max fuel range of 7000nm, which is 8055mi. In AE it's 8055mi.
The 777-200ER has a max fuel range of 9600nm, which is 11048mi. In AE it's 11025mi.
The 777-300 has a max fuel range of 8350nm, which is 9609mi. In AE it's 9609mi.
The "weird" numbers you're seeing are either:
1) estimated max payload ranges that don't match real-world payload/range charts (this is a game limitation as explained above)
2) stale data in older game worlds (range is never decreased in the middle of a game, so any data updates that decrease range will not take effect until the next reset)