Why Acetone does not behave like its computational values?
It seems that the results of the calculations are more or less fine and the OP just misinterpreted the NIST data. As I said in my comment above, NIST does not claim that $\lambda_{\mathrm{max}}=276 \, \mathrm{nm}$. Clearly only a small region of wavelength is shown on the graph and in the paper[1] referenced on the NIST page it is said (emphasis mine):


  There are two diffuse ultraviolet bands; the first at 2800 Å. is very
  weak with its oscillator strength $f \sim 0.0004$ and the second at
  about 1900 Å  is moderately intense with a maximum extinction
  coefficient $\epsilon_{\mathrm{m}} \sim 1000$. McMurry has identified
  the 2800 Å. band as a forbidden $\pi^* \leftarrow n$ transition
  involving excitation of a non-bonding $\ce{O}$ electron to an
  anti-bonding $\pi$ orbital between the $\ce{C}$ and $\ce{O}$ of the
  carbonyl group.


A high-resolution photoabsorption spectrum in the energy range 3.7-10.8 eV can be found, for instance, in this recent study[2] and is more or less consistent with the result of the OP's calculations.


Noel S. Bayliss, Eion G. McRae, J. Phys. Chem., 1954, 58 (11), 1006–1011.
M. Nobre, A. Fernandes, F. Ferreira da Silva, R. Antunes, D. Almeida, V. Kokhan, S. V. Hoffmann, N. J. Mason, S. Eden, P. Limão-Vieira, Phys. Chem. Chem. Phys., 2008, 10, 550-560. (available at researchgate.net)