The createIntegral() function returns the intergral
of the unnormalized RooGaussian function and serves
primarily to calculate the normalization that you need
to use RooGaussian as a normalized probability densitity function.
So the number you get out depends a bit on the formula that
was put in RooGaussian, but can in general be anything.
In general RooFit p.d.f.s are only normalized when you explicitly request a normalization. This makes it possible to use (multi-dimensional) p.d.f with more than one choice of normalization.
You can actually calculate the answer that you expected or wanted to get out, e.g.