Hi, dick_freebird:Upward is not the norm, but it's not necessarily an "error".
The right set of component curvatures could give you a
"non-classical" profile. However the real question always
is, are your models really accurate enough for voltage
reference design?
Modeling dudes will pretend they did a good job until
you show them data. Of course if you can't show the
data then you've got nothing.