Gosh... they're not "much better."

If they're ranked higher, DP rank 3 in US Open 2018 and Kevin rk 8 in Wimb 2018, it's precisely the result of their performance since last year... so what they did, including.... at the 2017 US Open.
It's pretty easy to point out because Del Potro in particular wasn't exactly a standout with consistent performances, getting injured quite often and falling in the rankings.
And if Nadal beat them both at the 2017 US Open, it's exactly the same thing when Djokovic beat them in the 2018 Wimbledon and US Open finals, nothing else.