Matthew had been postponing his evolution for quite a while and even though he didn't lose any levels because the system had automatically stored the excess levels that he got from killing all the creatures until now, it still means that Matthew was limiting the growth of his own strength.
It may be seen as counterproductive as his and the three kobolds' goal was to get stronger as soon as possible with the human army steadily closing in on them, but Matthew had a good reason on why he was delaying his fourth evolution.
