Personal Income Tax Calculation Tricks?

How to calculate income tax is quite straight forward.

If u look at any website shows u how to calculate and what r deductible etc.

But r there some tricks only Thais know?

Myself, being single and nothing deductible, owe quite a huge tax this year after the calculation.

On the other hand, my Thai friend who is also single and no deductible got tax refund.

What strange here is that when my friend filed tax via Revenue Department(?) website, it was 0 balance (no refund, no extra tax)

But when my friend filed tax via some tax consult, he suddenly got refund of several thousand baht.

I don't exactly know who this is. I'd assume some kind of tax consultant but not sure.

My friend said, everybody use some trick.

I don't know how it's possible.

Items that are deductible are very clear.

I don't know how else is possible!

Does anyone know what this trick is?

Thais around me r getting tax refund while I have to pay more!

(I heard they all sends some gift to this so-called consultant...)