Why this amount?

Hi there,

I have just received my assessment amount and can't work out where they got
the figures from.

I gave no pay slips and gave my gross salary and bonus details, backed up
with a letter from my work.

Why then is the amount they have used not what I have written, nor is it a
default £40 per week for (2 children). Any ideas where they have pucked the
"net take home" figure from?