This might be a language issue here, but at least in english this sounds a lot like “reduction (…) up to 90%”, which would mean 90% degradation / 90% protection. Now I have no idea what the original text says, but at least in English this is what it would mean.
This piece literally translates to:
“reduction in the performance of armor-piercing fin-stabilized sub-caliber projectiles [APFSDS] … up to 90% of the standard”.
I.e. “до” here doesn’t stand for “to”, but for “up to”.
It is, in fact, “up to” because you disregarded an important part of the same quote:
“oбеспечивается снижение характеристик бронебойных оперенных подкалиберных снарядов и кумулятивных средств поражения, в зависимости от типа, до 90% от штатной.”
… which translates to:
“The characteristics of armor-piercing fin-stabilized subcaliber projectiles [APFSDS] and cumulative weapons [HEAT] are reduced, depending on the type, by up to 90% of the standard.”
Thus these 90% encompass the maximum efficency when taken the entire selection of kinetic and chemical energy rounds, and not just kinetic specifically. It also doesn’t mean that round is degraded to 90% efficency, but their penetrative properties are reduced by up to 90% depending on penetrator and warhead.
The assumption it reduces, say, APFSDS like DM53 by 90% is wrong, but saying it reduces by 10% is about as much wrong because the original quote envisaged a much greater range of rounds, not limited to one specific, taken into account, and 90% is simply a number for HEAT rounds protection, but for APFSDS it will surely be lower, but not to that extent.
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