Why the paradox boys and girls, the outcomes of MD and DM are not the same?

In the analysis of the paradox of boys and girls referred to four possible outcomes before the first test: MD, DM, MM, DD

Why MD and DM different outcomes?
In my opinion the original outcomes: same-sex children (M), same-sex children (D) and heterosexual children.

Where am I wrong?
July 8th 19 at 16:38
1 answer
July 8th 19 at 16:40
Because what the child first, some the second.
The birth of a child is unrelated to the event.
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