The calculator on the government site is, of course, correct.
But the lowtax.net site is also correct (though it is for the 2009/10 tax year) - you simply haven't understood it. What it is saying is that your marginal tax rate on earnings over (allowances + $120,000) would be 17%, not that you would be taxed at 17% on your entire income. (Again, those were the figures for the 2009/10 tax year).