In a tank circuit why can be the energy across the inductor and that of the capacitor not be equal?

When an LC tank is excited at the resonant frequency, the energy across each will be equal (but not necessary equal at a given moment in time). If excited at a frequency other than the resonant frequency, the impedance of the inductor (wjL) and capacitor (1/wjC) will not be equal, therefore energy across each will be different.