Discrepancy of trypan blue and BrdU incorporation

[GisLove]

I have assessed the effect of drug on stellate cells using trypan blue and Brdu incorporation method.
Since trypan blue is to test the cytotoxicity and BrdU incorporation labelled the proliferate cells,therefore,I assumed the IC50 obtained should be not much difference.To my surprise, there is like 6 fold difference,with IC50 from trypan blue is smaller than IC50 from BrdU. Can anyone explained the theory behind?Thank you.