Even if you take into account they took out a yonkou, it's still 2 vs 1 compared to Luffy's final 1 vs 1 match. Yeah, Luffy still got help, but hey, Kaido is believed to be stronger than BM.
The only excuse is that people outside Wano didn't really know what actually happened, so they just grouped all the feats together. Well, now we can see that Kidd got no diffed and Law got mid-diffed, while Luffy is setup to beat an admiral. Unless Luffy also suffers a major defeat there is no way those bounties should match.