Can you guys enlighten me on this age's intel gathering?

I've always read in the forums something like this: 20% + 1 thieves = max 3% error.

I just took an intel with 20% + 3 thieves and my result for a province's def was 2769.

A tact took his intel and showed 4928. The diff is outstanding. I still have my page there untouched just to be triple sure I did not send the wrong amount of thieves, etc.