My experience with timesheets is that they must add up to the hours you're paid for (full-time salaried work, not hourly consulting). That more-or-less guarantees that timesheets are made-up.

My first salaried job was as a salesman; we incurred expenses, which we could claim back. It was explained to us that the system was there to ensure that we weren't out-of-pocket; claims didn't have to actually be true, but you did have to have incurred the expense.

I fairly soon realised that this was a lie; the expenses system was really a salary-augmentation scheme, and you were expected to inflate your claims. Everyone was at it.