There's a lot on the line this time.You don't go from staggering around like an idiot after using a technique, AT FULL STRENGTH, to being worried about your chakra levels, using it twice, and another technique in-between...in a very short span.
During the fight with Sasuke, which was very shortly before the war, using kamui rendered his body useless for a period of time.
He didn't improve that much in that short of a span.
Remember when Sasuke was training his chidori? He could only spam 2 or 3 chidoris. Now what? He can spam how many he likes. Maybe because he learnt how to control his lightning nature better. Or did his chakra levels raised? Or was it his manipulation of chakra?
and your the athority on kakashi improvements i guess? if you have ever been a part of real sports in you life you would be aware that stamina can be improved a great deal in a short time if your willing to work for it
Comparing Naruto to real life? Oh dear lord.
And Kakashi didn't have any time to improve...when he got back to the village, he had to prepare to become Hokage, then Tsunade woke up. Very, very little time in-between that and the start of the war, which given he's a general, he had to prepare for as well, leaving him no time to train.
Kakashi himself said when he made the clone for the first raiden that he was worried about his chakra. Since he made that statement, he's used:
Raidenx2
Raikiri
Kamuix2
And that's after having to fight the 7 swordsmen. Use some common sense. The only reason he can still be standing right now is if when in the kyuubi cloak, their chakra was restored. Otherwise it's the perfect example of plot saving them.
Of course this is because of the plot. Everything is about the plot. Please name me one manga where an important char. didn't surpass themselves. The answer is NONE. I'm new here but I'm starting to think that a lot of members on this base don't read many mangas.
99% of battle mangas show char. going behond their limits in each and every fight and that's a FACT. People were crying because they didn't see Kakashi vs the swordsmen. Now that he's fighting against way better opponents aka TOBI and JINKS and doing well, you guys are complaining :sy:. I'm starting to think that a lot of fans from america are getting tired of the Naruto series.
You guys would try to find anything to degrade it mainly because it's not going the way you want it. Sure, their are lots of things that don't make sense. This is the case for every manga out there.
To answer the question, they get better with training. Simple!!!