Lipo battery voltage drop

Bro, is it normal for a 7.4V 25C 4500 mah lipo battery to drop the voltage to 7.2V when it is running half empty. I can understand when it is fully charged voltage should be 8.4V but would voltage drop so much when it is half drained? I noticed it after I overdrained it once to 6.4V. Lipo is supposed to
maintain the performance all the way till the voltage is low, does that mean my battery is less'explosive' now?

Philip