Hi DP,
that is because you use NIntegrate, which integrates numerically, i.e. it takes finite precision in the process. Therefore, Mathematica indicates that this is a numerical result.
If you use Integrate instead:
Integrate[x^2 + 5, {x, 0, 3}]
you get 24 without the fullstop, i.e. "infinite" precision.
Cheers,
Marco
