Application of IRS pub 526 charitable contribution limits

I am planning some significant charitable contributions, and I am confused by the way TT applies IRS pub 526. Here is what I observe as I run three scenarios.

% of AGI as             % of AGI as           Deducible

Appreciated            cash

securities

30                              20                         50

0                                60                          60

30                             30                          50

Can anyone explain why in the third case [donating 30% of AGI as appreciated securities and 30% of AGI as cash) only 50% of the AGI is deductible?