Despite still remaining the team to beat in F1, Red Bull have struggled to find the consistency this year which has served them so well in recent campaigns.
Though top of the drivers' championship, Verstappen has been some way off his best, including at last weekend's Hungarian Grand Prix, where he finished P5.
The form of his team-mate Sergio Perez has also allowed their rivals to close the gap in recent outings, with McLaren in particular emerging as serious challengers to the Red Bull throne.
Horner has been impressed with the British team's progression, and believes their car's ability to perform at optimum level across a variety of circuit configurations suggests that it is currently a more versatile machine.
“I think their car is in a better window than ours at the moment, it’s perhaps easier or a broader window,” Horner admitted, as reported by MotorsportWeek.com.
“Our window seems to be very peaky, and that’s what’s making it difficult for the engineers, difficult for the drivers.
“I think we have to take the balance of the car and make that window broader as it’s so critical on temperature and all the relevant factors.
“And I think that’s something that the team are very aware of and working very hard on.”
“McLaren obviously have a lot of pace at the moment, they had a big score [at Hungary], and we need to start scoring big points from next weekend onwards,” Horner added.
“I think we’ve got more performance to bring, so as I say I think we need to expand that operating window for the car."