That's because his PRNG is not uniformly distributed. I try to show it in the ascii graphs below:

Uniformly distributed:

   start  avg   end
    _____________
   |             |
 __|             |__

Adding to start/end changes the avg:

       start  avg   end
        _____________
       |             |
     __|             |__

Normally distributed:

  start  avg   end
     __________
    /          \ 
 __/            \__

Changing only start/end may give strange results:

       start        end
        ________
       |        \
     __|         \_______

[download]

In reply to Re^5: Skew normally distributed random numbers by fglock
in thread Skew normally distributed random numbers by Anonymous Monk

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