Isotope labeled experiment analysis

I'm analyzing a dataset generated with LC-MS from cells that were fed $\ce{^13C}$-Glucose. There are 3 groups in total each with a different media conditions. My goal is to identify differences between these groups. When I look at the distribution of labeling (MDV) I don't see any difference across the different metabolites but when I look at the relative abundance I see large differences. How can I interpret this, when the labeling patterns are the same but the relative abundance is significantly different? Thanks.


