Well.
I have an explanation.... Poor but anyway.
Don't give the Amerikanski Russian engines.
Those engines are russian built from the 60's, they are
NK-33 Engines which are MODified and called Aerojet AJ-26... And you know what modifications mean.
They are actually closed loop engines meaning the turbopump used to
thrust fuel into the chamber consumes fuel, the exhaust of the turbopump goes INTO the chamber which gives the rocket additional
thrust and cleaner burning. This also means it's much more likely to blow up (in the past american experiments failed again and again) but the russians perfected the engines and metals used (because oxygen corrodes metals). The
RD-180 engine from the russians is one of the best rocket engines, consumes little, high power and reliable. They are used in US rockets today.
American engines usually were open loop meaning the exhaust of the turbopump got lost during firing so less power and not so clean burn but more reliable.
Orbital sciences has almost 0 experience on those russian engines, probably sooner or later it was going to happen.
Spacex on the other hand has their OWN developed Merlin engines which are quite good.