Minimum warhead arming distance.
That being said, I tend to agree that the Russian army offensive is proceeding according to plan. If the Russians are suffering heavy losses in places, it is directly due to direct intervention by US and British special operations forces, fighting on the ground with Ukranian troops and the effect of covert US airpower targeting Russian units, also directly. The Russians did the same thing to us during the Vietnam War, as we did to the then Soviets during their invasion and occupation of Afghanistan back in the 80's. Superpowers love to test new military equipment against each other's forces indirectly, during all such proxy wars.