I'm talking about how its implemented because it directly affects how many points you should put into decoy, if you're going a decoy build. If points above say 16 into decoy would be worthless, since they would not be adding any extra resists (because base resists are capped at 85 on her), then you could spend those points elsewhere since all they'd give you is +life onto valk, which isn't very useful.
Yes Maximum Resists & Normal Resists are different, but thats not the point. What is going on is that the game is capping your
base resists, using another maximum resist cap.
Specifically, you have:
Your base resistances
Your base resistance cap (only used when the function instantiates a new valkyrie)
Your +resistances from statebased effects
Your maximum resistances value.
In this case:
Base resists = (decoy + valk) * 2
Base resist cap = 85
+resists from items = whatever items she generates
Maximum resists cap = *infinite* (100%, because negative damage will be discarded)
Small effects like these can have enormous impacts depending on how they were coded. For a specific example, look at magic damage reduce- the way that it was implemented makes elemental damage of any kind (sojs, spirit sword) on a kickersin in LLD utterly terrible, because it will actually make your attack do 0 damage because the MDR will "splash" over to reduce your physical damage with the leftover reduction from the elemental, and reduces each element once. By knowing this bug exists, you can avoid making a kicker who will deal 0 damage per kick to anyone wearing vipermagi/string, which is most builds in LLD.
In this case, the programmers would have had to do something extremely backwards and stupid for the valkyrie resistance implementation to be anything different than:
A) Base resist is capped at 85% when unit is created, using max((decoy+valk) * 2,85)
or

Base resist only includes the valkyrie skill level, and decoy is applied as a statebased effect.
However, in the case of B, any valkyrie spawned with level 40 valk / 20 hard points into decoy would result in a 4x immune valk regardless of gear. This simply is not the case- it requires gear to be immune still. So that is not how they did it