Asked if he felt Mercedes was taking the hit with penalties to trade for the improved performance of new components, Horner replied: "I think they are weighing up that risk versus reward.
"It is very unusual for Mercedes that have been obviously the benchmark for reliability for the past six or seven years so it is unusual they are in this situation."
Mercedes is in its closest title battle in the turbo-hybrid era, with Red Bull attempting to reclaim both F1 championships for the first time since 2013 and Horner suggested that pressure may be having an effect on its rival's engines.
"I think we are pushing them hard this year," he said. "We have really thrown everything at his season and pretty much every race we have been able to give them a hard time.
"When you consider how dominant they have been the last seven years and that this car is largely a carryover of last year's car, which was arguably their most dominant, is a testament to the team that we have got ourselves to this position where we are challenging them with six races to go."
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