I can't give you the exact biological reason for it, but I have seen it alot even in my own cats, where in the very beginning they react really hard to those first couple of doses after not having insulin for awhile and then it seems their bodies get use to the extra help and they go higher and stay there until you up the dose. Then again it also could be a bounce off those blues that send him to that 400 and those can take up to 3 days to clear, which is why we hold a dose for so long to allow them to get use to it and show us exactly what numbers they are really going to give us.
Right now there just isn't a lot of data to know which it is, so he is either clearing a bounce, or just has a little bit of what we affectionally refer to as New Dose Wonkiness. Once he has been on insulin for awhile it will become easier and easier to tell which is which, as he should establish a pattern that will let you know. Like if every time you change doses on him he jets up for a few days and then slowly gets lower and lower then you will know it is just NDW, but if you see him drop either really low, or at least lower than he has been in awhile in a cycle then flies up for a few days that is a bounce.
The thing to remember (and this was a tough one for me in the beginning) is that they can bounce off any number. They don't have to go super low to bounce, just lower than they are use to. So if he was say staying all the time in the 300s and suddenly goes all the way down to the low 200s while not dangerously low, it still feels weird to him and his body wants to go back to what it has learned as normal, so it releases stored sugars and counterregulatory hormones which raise him back up. Think of it like when you have been driving on a highway going 75 mph and suddenly have to slow down to 45mph to go through a town, while 45 isn't particularly slow, it still feels like you are crawling along because you have gotten use to what it feels like to be going 75mph.
Mel, Maxwell, Autumn & The Fur Gang
