confused

[JBee]

According to ancestry my son's dna results are:-

1st cousin

My half sister with 787cm over 30 segments is a 1st cousin
Her/my niece with 706cm over 18 segments is also a 1st cousin

2nd Cousin

My half sister's son/ my half nephew has 412 cm over 20 segments
- a lot less than his full cousin.
.................................................. ......

Now how significant is it the number of segments?

Most of the results are 1 or 2 segments and 4th cousins

but now I have found a match of 36cm over 6 segments a 4th - 6th cousin? As there's no tree I can't fathom out where she fits in except by seeing which family she shares matches with.