The formula gives you rough estimate. Here are some cases where it can be biased.
Imagine that according to this formula you are behind.
If you got behind by being unlucky, then this formula will predict a lower chance of your victory than what is actually true (Ex. my game against ncscswitch where I recovered from what I thought were long odds of around 1 in 20 - the long odds weren’t as low as I thought they were.)
If you got behind by bad strategy, then this formula will predict perhaps a roughly equal chance to that which is actually true (this is my guess - I’m not entirely sure about this).
If you got behind despite being lucky, then this formula will predict a higher chance of your victory than what is actually true.
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If you take over a game that is behind, the average player (taking over a losing side) will have a higher chance of victory than what this formula predicts (since players who get behind in the game aren’t as good, unless they got behind due to bad luck).