The game doesn't have to calculate your RTS behaviour, merely a reasonable RTS behaviour. If you want it to use your behaviours and choices, enter RTS yourself.
If a 20% chance to win a battle means that you can not win, then it is not a 20% chance to win. One out of every 20 or 30 is a 5% or 3% chance. An 80% chance to win is not a guaranteed win, nor is a 90% chance to win, nor should it be.
You are not forced to engage in every battle. Giving results that ignore the chance to win would do that much more than that chance being accurate. Remember in an earlier beta when the auto-resolve was not accurate, and people with a low chance to win would always enter RTS because they could win against the AI that way, or even the odds with another player?