There are few things more heartbreaking than to have played this game very well, to have established paying properties out in front of the computer players, only to be cheated round after round, hitting every costly spot on the board, while the computer opponents skip happily over your properties turn after turn.
Having played hundreds of games against the computer, it's more than obvious that the different skill levels are accomplished by rigging the dice. At the end of each game, a table of throws shows a natural distribution, more or less, but the timing of when those throws either benefit or hurt you is controlled, without a doubt. For the "hard" level. it's set on a 1/2 - 2/3 bias; whereby you will make good throws 1/3 of the time, while your computer opponents have good throws 2/3 of the time. Hundreds of games have taught me that.