No alternative explanation needed. The objective fact is they received 1 proposal from a woman out of 250. If you want to claim that ratio proves the process must be biased, you need to show some evidence, not a bogus math formula which was flawed from the beginning.

As other repliers have already stated: a) you chose a flawed survey to get your "10%" figure, b) the percentage of "women in open source" is far different, for a variety of reasons, than the percentage of women who are likely to submit a proposal to speak at a PHP conference in Germany, c) you dismiss any possibility that unusual ratios could possibly be due to reasons other than bias (is lung cancer biased because it overwhelmingly affects older people?)

You've offered zero fundamental evidence of bias (i.e. they filtered their mailing lists, they adjusted female speaker ratings, etc), other than the math doesn't work out the way you'd like, which isn't surprising given the invalid inputs and assumptions you started with.