I have started playing around with the "Odds & Outs" software (v 1.00.10d)
and I believe it is counting the backdoor straight outs incorrectly.
Example
You: 4
5
Opp: 3
A
Board: K
9
3
I believe we have 3 fours + 3 fives, a backdoor flush draw with spades (1 out), and a backdoor straight draw (345). Since there are no gaps in the backdoor straight draw we can count it as 1.5 out.
That gives us 8.5 outs. If we multiply it by 4, we get 34% equity. Pokerstove actually gives ~ 30%, but it's close enough.
However your software says that I have 11.5 outs and 46%~49%.
The answer shows 6 immediate outs (no surprise here) and 5.5 backdoor outs:
26, 2A, 67, and
So basically it counted 1.5 out for every possible runner-runner straight combination, which in this case seems to be a huge overestimation.