Fascinating. Shouldn't the end cost be something straight forward?
New End Cost = Base End Cost + EndRedux
Its very straight forward and is accurate to the description. Using the current calculation you've shown, its not reduced by 2.5%. If you did that in class you're get it marked wrong.
The point of showing numbers like that is very important because its how people crunch numbers for builds. If you did 5 sets with this then that's 12.5% and that's a hefty reduction of end. The player would be expecting that end cost formula above to be used and would be calculating their build accordingly. They are expecting to only have 87.5% end cost on their abilities instead of the 88.88~ it would otherwise be. That's over a full % lost with an incorrect formula causing a 10% difference in expected results o.0 The expectation vs reality gets much larger apart when you figure in other end redux the build may have.
Its fine to use that formula though. I'm sure there are reasons its used, probably concerning diminishing returns on things. The issue is the description for end redux on that set bonus, or any other incorrectly displayed end reduction. It would be better to say "reduces end reduction by a small amount" or actually say 2.44% instead of 2.5%. This way they know to check combat attributes to see the proper number, or they'll actually have the proper number to begin with.