i've been looking at it for a while so i'm starting to confuse myself.

let's say a couple plans to have 4 children. these are the possible combinations:
bbbb
bbbg
bbgb
bgbb
gbbb

gggg
gggb
ggbg
gbgg
bggg

bbgg
ggbb
gbbg
bggb
bgbg
gbgb

that means that they have a 6.25% chance of having all boys and a 6.25% chance of having all girls.
it also means they have 37.5% chance of having two of each, in any order.
plus, they have a 50% chance of having one of one gender plus three of the other gender.

i don't understand why there's more like to have 1 of one gender plus three of the other gender rather than 2 of each. if the probability of having one is independent on the next one, then each time it's 1/2. it could be i'm making a math error, but i'm not seeing this...

There are more possible combinations of "three of one and one of the other" than there are for 2 of each. If you would specify 3 boys and 1 girl, or 3 girls and 1 boy, there are fewer possible combinations.

Those are permutations, not combinations....