Hours worked on a Interim contract

I just did my first job for Tempo-Team Interim in Brugge.

On the contract it states that I worked 3 hours, but in fact I worked 6+ hours.

I just went to their office and they tell me that the three hours is what they have agreed with the client, but I will be paid for 6 ?

Is this legal?

Why cant they just state the hours I worked?

It's also more complicated, I am British and applying for my residence ID and have to show the commune that I am working at least a minimum number of hours in a week regularly.

What say you?