a unused s9 is $850
a unused s15 is $1450

s15 is 28thash = 2x s9(14thash)
s15/2=$725
s15 wins

also s15 is 1.6kwh. so s15/2=0.8kwh,   unlike an s9 which is 1.3kwh
s15 wins again

what people dont realise is what happened in october
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this is normal hashing rises prior to october where more s9's are steadily added
this is them not adding more S9's
this is the initial replacing 1s9 for 1 s15
thus going back to normal steady rises but where they would only turn on 1 s15 when they would normally turn on 2 s9
this is them turning off some s9's and swapping 2s9 for 1 s15 (flat swap)
this is them realising they could actually take 8s9s out and only put 3 s15s and still win blocks while not having to run as many rigs

you can see on the hashrate charts the curve. though stretched out and zoomed out you dont notice the curve as much
forgive the colouring below. but i grabbed an earlier image i done to show some other topic something

the smaller image within the image is the 12 month hashrate. the larger image is a zoom on the october/november curve

looking at the image within the image. the blue highlight is normal action. the pink highlight is the initial purple flatline
the yellow highlight is the green orange purple red curve explained above