Proper motion of stars

I am comparing declination values obtained by Tycho Brahe (1601) with values obtained using Stellarium and other planetarium programs as well as values calculated by myself correcting Hipparcos-2 values for proper motion and precession. For 15 stars investigated, the values obtained with Stellarium differ from 6" to 1' 54" compared with the other programs and my calculations.

As the object of my investigation is to determine the accuracy of Tycho Brahe's instruments (generally believed to be less than 1'), an error of more than 20" in declination obtained by the program providing the base line data is not acceptable.

Stellarium corrects for proper motion so this is not the cause for the discrepancy.

What may be the cause for the differences?
If necessary, I can provide actual values if necessary.

Thanks
Ole Schou