Yeah man every network has a different standard and threshold. CW isn't as big as the other major networks so their standards are different. Like Arrow and Flash are hit shows but if either one of them were on CBS it'd be an issue.
What's crazy is when the CW was the WB (they later combined with UPN) they used to get triple those ratings. If Arrow/Flash came out 10+ years ago the average ratings would be in the 7 millions.