The 390 isn't much more than a speed bumped 290 and the 480 isn't a huge margin faster than either, unless we cherry pick results. With less shaders and a drop in core clock (it needs it to drop in line with perf/watt expectations), I wouldn't be surprised at 280/380 (x) speed
I am not sure why you are accusing people of cherry picking things:
https://tpucdn.com/reviews/AMD/RX_480/images/perfrel_1920_1080.png
That is over 16 games.
The RX480 has 12.5% more shaders than a RX470 and tends to throttle over time to under 1200MHZ - reviews have shown it settles down to around 1150MHZ to 1170MHZ. If you do some quick sums of 2304 shaders at that speed it equates just above an R9 390,which fits with what TPU sees,ie, 2516 shaders at 1050MHZ vs 2304 shaders at 10% more clockspeed.
So even if it has a lower boost clockspeed,the RX470 will still be closer to an R9 290. I expect the RX470 to be around 1.1GHZ to 1.15GHZ and I suspect for cost reasons AMD will use the same reference PCB and cheapo cooler from the RX480,and I suspect due to the lower boost frequency and cut down GPU it will boost more consistently too. Could be wrong OFC.
The R9 390X has 10% more shaders than a R3 390,yet is not 10% faster according to TPU.
If you looked at the R9 290 series for example,they gained massively in performance/watt closer to 900MHZ,as opposed to around 1GHZ where the cards were essentially overvolted.
Edit!!
Even if you look at the R9 380X and R9 380 they are around 10% different in speed.
The same as the R9 280X vs the R9 280 and the HD7870 and the HD7850.
Even at a 15% difference it would be closer to a reference R9 290.
Second Edit!!
Even at 20% difference that would make the card in-between a GTX780 and a R9 290 overall.