fill in
I cannot fathon why " fill in " is correct in the two following sentences

fill in a form

fill in your personal information in the chart

I mean why can it be followed by "what you use to fill something in " and by "the thing you fill in"?

Thank you very much

Question

fill in
I cannot fathon why " fill in " is correct in the two following sentences

fill in a form

fill in your personal information in the chart

I mean why can it be followed by "what you use to fill something in "  and by "the thing you fill in"?

Thank you very much

Answer

An interesting question!
I think you will find English has many similar peculiarities.
I am not sure of the exact grammatical explanation, but "fill in a form" and
"fill in your personal information" both sound correct.

Answer

I don't really understand your question.