You do understand that the "time remaining" number is estimated based on your current power usage, right?
So if your MBA is drawing twice as much power right now (because, for example, the CPU is working especially hard), then the "time remaining" right now should be roughly half of what it was previously. And if your CPU power usage decreases and the MBA starts using half as much power, then the "time remaining" should jump back up. If your power usage varies up and down a lot, the "time remaining" should jump around accordingly.
If you want a totally accurate "time remaining" that tells you with precision exactly how long your battery will last, you need to wait for the new BatteryPrescience feature, which will look into the future and see exactly when the battery's going to run out based on your future usage.