I’m never quite sure what they mean when they say "extended release" vs "controlled release" vs "sustained release". I think "extended release" is the same as "sustained release", i.e., it follows first order kinetics (whereas, "controlled release" follows zero-order kinetics). Basically, the levels in your blood will still fluctuate–just not as fast as they would with the regular formulation. Because of this slower behavior, the drug can be prescribed once/day instead of twice/day.

Why did your doctor prescribe it twice/day? There are two possibilities:
(1) it was a mistake and the doctor meant for you to keep taking the regular Keppra, or (2) since you are at the max dose already, your doctor may feel better breaking it up into twice/day to help keep your blood levels more constant throughout the day. Keppra is not something I prescribe in my field, but I’ve seen the same thing done with Lantus (insulin glargine) dosing in diabetics. It was meant to be a once/day dosing, but some doctors prefer to break it up to twice/day because they think it helps maintains a more constant level throughout the day.

The only way to know for sure is to ask your doctor why he prescribed it that way.

In epilepsy treatment initial dose is small and after few weeks the doses are increased.

So your information, ‘extended release’ doesn’t mean that it is once daily.