It could be because of call playing due to the personnel available.
The Broncos actually have one more TD than the Texans (even without playing an OT game - though the Texans didn't score a TD in either).
And Keenum had a few very close balls and even call-back due to penalties.
It was also that the Broncos feel like they can punch the ball in near the goal line - or sometimes they can score on a run beyond the redzone.
Don't forget that Keenum was very efficient last year in the redzone.
https://www.numberfire.com/nfl/lists/19037/the-5-most-efficient-red-zone-quarterbacks-in-2017
The guy that can help himself the most is DT,
if he can limit his drops.
He had more targets with the Broncos than Fuller with the Texans.
If he had fewer drops, his catch rate could equal or better than what he had with Manning.