There's a storage shed/insulin depot under your cat's skin. It has first dibs on the insulin. Once it is filled, the insulin that you shoot doesn't have to go into the shed. It can be used. The shed HAS to be filled before the Lantus or Levemir gets to use the extra on cat's BGs. So by skipping shot, or waiting too long to shoot, or giving a fur shot... the cat starts dipping into it's shed and the next shot or shots have to partially go to refilling shed before they can work on BGs.
When you get impatient and overfill the storage shed by upping the dose too fast, you get into a situation where at first it looks like nothing is happening because the shed is still filling up. Then you reach the point where it is totally full and yet you are still giving a dose that's too high. Now suddenly none is going into shed, because that is full. It is all working on the cat... and the numbers drop out on you.
Worse case scenario is when you over fill the shed/insulin depot... get very low numbers... then empty out shed by dropping dose too much... or having to skip the shot. Then you'll see high numbers from the combo of the effects of rebound and an empty storage shed/insulin depot. Most react by bumping the dose back up which overfills the shed/depot again resulting in a roller coaster ride to nowhere. Not the way to work this insulin.