This is normal C++ behavior.
The small deviation shouldn’t have any impact most of the time, but it’s something to be aware of. In most UE functions dealing with float/double, there is a Tolerance parameter (with a sensible default value) to mitigate the deviation, for example in FVector comparisons :
If you are comparing floats/doubles directly you should also take that into account :
float A = 0.1f + 0.3f;
// Don't do this
if (A == 0.4f)
{
//...
}
// Do this instead
if (FMath::IsNearlyEqual(A, 0.4f))
{
//...
}
// Or this
if (FMath::Abs(A-0.4f) <= UE_KINDA_SMALL_NUMBER)
{
//...
}
When it comes to displaying floats and doubles to screen, use a format to automatically handle rounding to the amount of decimals you need. Having a deviation at the 10th decimal isn’t gonna impact the number you display with 2-3 decimals.