Burrow's first 2 games: 'making his own mark in the NFL'

[Bengalholic]

I thought this stat was interesting:

On 1st downs, Joe has only completed 20 of 43 passes (46.5%) with a QBR of just 63.
But he's a combined 40 of 54 on 2nd, 3rd & 4th downs (74.1%) with a QBR of around 95.

What's creating such a big difference in results on 1st down passing plays compared to all other downs?