Planned obsolescence. Nvidia's performance numbers in benchmarks often include overclocking numbers. This makes their cards appear faster to non-enthusiasts who will never overclock all the while not actually providing that performance because they don't overclock.
It is fine to include overclocking in benchmarks but it would be misleading to assume that allot of people are going to be taking advantage of it. I'm personally not a fan of it being the deciding factor of a conclusion unless your website is specifically for enthusiasts.
Given the information from the benchmarks above, I would recommend the R9 390 over the 970 at the same price. It's faster out of the box and matches the 970 overclocked. FreeSync should be considered as well, being cheaper and having wider adoption. Unless you are just buying a stop gap card, I see no reason not to pick the R9 390 over the 970. Right now though GTX 970s are cheaper 2nd hand in the US, so it's really whichever is cheaper unless you already have a FreeSync monitor.