i don"t understand why the same number cell and celltype in seurat object lead to different averageExpression() result

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Idents(scRNA)="combined_celltype_add.malignant.normal"，
exp1=AverageExpression(scRNA)

scRNA_subset=subset(scRNA,combined_celltype_add.malignant.normal%in%c("Malignant_cell","Epithelial_cells"))
Idents(scRNA_subset)="combined_celltype_add.malignant.normal"
exp2=AverageExpression(scRNA_subset)

even the number of epithelial and malignant cell in scRNA and scRNA_subset is the same:

result of exp1 and exp2 as follow :

i wonder that why thethe averageexpression result of "Malignant_cell","Epithelial_cells" in exp2 is different from exp1?please who know this ,i know is simple but reslut is uncredeble