Seurat re-clustering a cell subset but cell identity numbers are not completely showing up

Hi,

After running T-SNE plot on dataset, I tried to extract a specific cell type then do reclustering. But I found that the some of the cell identity IDs are skipped in the reclustered T-SNE plot (0,2,3,4,6… instead of 0,1,2,3,4,5,6):

I cannot figure this out nor find help from other website resources. Could anybody provide some help on how to interpret this?

Thanks so much!