I'm quite sure this is either a small or just plain stupid mistake but my anxiety is getting the better of me. when calculating exhaust velocity this is the equation used:

[![enter image description here][1]][1]


however I keep getting higher values than that listed, specifically on [Astronautix][2]. I should mention I am solving for monoprop HTP-85%, where M = 20.7, k = 1.34, Pc = 10 bar, Pe = 1 bar, Tc = 910k and R is universal gas constant. I suspect that it is just the difference between theoretical and actual performance however 1128ms^-1 is very different from ~900ms^-1. Is there something I am missing or am I just using the formula in the wrong context? 

  [1]: https://i.sstatic.net/TJ34R.gif
  [2]: http://www.astronautix.com/h/h2o2.html#:~:text=Temperature%20of%20Combustion%3A%201%2C165%20deg,%3A%20H2O2%2D90%25%20monopropellant.