Nucleophilic addition - Propanal vs Benzaldehyde

According to my school textbook, carbon atom of the carbonyl group benzaldehyde is less electrophilic than that of propanal because polarity of the carbonyl group is reduced in benzaldehyde due to resonance.


I can't understand how. If I draw the resonating structures, the oxygen atom takes the delocalised electron and not the carbon atom. So, resonance shouldn't affect the electrophilicity of the carbon atom. Moreover, benzene being a -I group, induces a partial positive charge on the carbon.


What is the correct reason?


