area and height difference

By what percentage are they different? That's really the point of internal standards, to account for varying amounts in each run so you do get a good calibration curve.

Like 50-200%

Do you know how simillar should the internal peaks be? How much can those differ?

Do you have any clues why my calibrering curve is still good even though Internal standard peaks are so different?

That's the plus to using internal standards. Yes areas are different but so are the areas of analytes, and apparently close to the same ratios compared to the IS over each run. Thus a great curve. 

I did some identical samples and the area% or high% between the compound of interest and my internal standard was about 70:30.

Between the vials the difference between IS was bigger- for example in the 1st sample the area was twice as big as in the 2nd sample.

What do you think?

I did some identical samples and the area% or high% between the compound of interest and my internal standard was about 70:30.

Between the vials the difference between IS was bigger- for example in the 1st sample the area was twice as big as in the 2nd sample.

What do you think?