Borates don't lower the CSI (not significantly). What they do is increase TA somewhat where 50 ppm Borates at pH 7.5 contributes 5 ppm to TA. So what you are seeing in The Pool Calculator if you aren't changing the TA is the effect of lowering the TA after Borates are added. If you instead increased TA by 5 ppm when adding 50 ppm Borates, then that would not change the CSI (at pH 7.5).