What type of splines do epics use in UE4?

question:

what type of splines do epics use in UE4?
I thought it was cubic but it is not …

using the example of constructing a circle with a radius of 1

tangent of cubic splines of length = 0.5522847498307933
//(-4 + 4*sqrt(2))/3

tangent in UE4, length = 1.66
//the value is matched visually, what is spline math?

???

https://en.wikipedia.org/wiki/Cubic_Hermite_spline

0.5522847498307933 * 3 = 1.65685424949238‬
VS
1.66