It appears that way because that's the data you have recorded, but it doesn't tell the whole story. You said when she was diagnosed, her bg was over 400 and the fructosamine came back at 510. Fructosamine is an average over a few weeks, so that means upon diagnosis, her average glucose was in to 500s! You also changed food to low carb right away, I'm sure this helped to bring the numbers down a bit, but she clearly still needed a little more help with insulin because of both her numbers as well as developing keytones. Also, when you changed food and had started the Vetsulin, you were having trouble getting her to eat. Less eating usually equals lower BGs.
Bella has done well with the slightly higher, though still low carb food. This could also cause higher numbers, depending on how carb sensitive she is. You want the higher carb low carb food so you can get more insulin in her to combat the keytones. It also sounds like she is eating better now, so that changes the insulin needs. Had she lost and since gained back any weight? That can contribute as well.
Now you're at the stage where you are waiting for times to safely increase, until she reaches a level of insulin that keeps her in good numbers below the renal threshold. She has come a long way in a short time just by the fact that she's feeling better, eating better and has had a nice stretch of negative keytones. It's hard when the numbers don't reflect this, but they are not their numbers and we can't ever forget that.
Rather you are following SLGS or TR, the rules are a bit different for Bella because when keytones are involved, the rules yield to handling and staving off the keytones. You are doing great, and Bella will likely need an increase soon, but what a long way she has come!