I'm so confused...

Tested three DNS servers as our internet is surprisingly unresponsive for a 40/40Mbps connection (p.s. figures are 3 run averages)

Best website loading speed: Google's DNS (clearly faster, over half as long compared to others. ISP is second fastest whilst cloudflare is slowest)

Best download/upload speeds: ISP's DNS (ISP's DNS 38/36Mbps, Google 17/30, Cloudflare 18/28, no idea why external DNS servers are slower)

Lowest ping on speedtest.net: Cloudflare/Google's DNS (ISP's 30ms, Google & Cloudflare 7ms) 

Take the data with a grain of salt as it was tested with a load of other variables but still, what? How does changing DNS servers affect download speeds? It affecting responsiveness and ping is not surprising as that's effectively the sole reason why you change what DNS server to use but download speed being affected?!