Why linux on virtual box showing 2 hours battery remaining ,while main OS windows 7 showing 2:30 hours battery remaining?

Because they use different remaining uptime prediction algorithm and because they're not sampling power consumption at the same time.

Battery itself doesn't report how long it will last. Laptop's circuitry can, however, inform the system of current energy consumption and remaining battery charge as reported by the battery. OS will sample these data periodically and estimate how long the battery will last under current usage.

The exact method of calculation differs between operating systems, though. And if you'll install BatteryBar on Windows, it will report different remaining time than Windows's power icon which is next to it for the same reason.

We also have to take into account that power usage can fluctuate very rapidly as programs are doing their job and this can affect estimations. Sampling at different points in time can lead to slightly different results.