So its just like this.

Institution received dividends say ex. 100 in which 15% of that are withheld and has an attachment of tax cert. So net of 85.

Then eventually they, will file income tax return Assuming their income tax rate is 20%. The computation would be
Income of 100 multiply by 20%, that would give a 20 tax payable.

But before that, they will deduct instead of 15 tax cert above, they will deduct 25. And that would give them a tax refund of 5 (25-20).

The tax authorities can only detect such wrongful info, via from dividend issuer, income tax return coz they're gonna check the tax credit.