Hello,
The DS1621 is capable of higher resolutions.
By reading out the 8 bit temperature (not the 9 bit), counter and slope of the DS1621 and performing a calculation you can get a higher resolution (this can also be read in the datasheet).

I’m still working on the calculation using signed integers and I hope to test it tomorrow.
Using pen and paper the calculation seems to work.
Greetings,
Windt H.J.