Is this normal....lipo

[speed demon]

I bought a lipo awhile back and have been running it for awhile. Its a 3s 18c 4000mah. For the first time I ran it until the speed control shutdown because of low volts. I went to charge it will my Imax b5 charger and it put 4600+mah into it. I charge some of my buddies 4600nimh bats and I can only put 3800mah into them when they are dead. Is the count on the charger accurate? Do lipos normally hold more then their ratings? Thanks.