Interpretation for increase of counts of non-zero values in single cell ATAC-seq samples ?

Question

[Alexander]

105942 cells and 228942 DNA regions for ATAC peaks. Cells were sequenced at days 2,3,4,7.

Let us count number of non-zero elements for each cell. And make a plot of it.

We see some clear patterns of day dependence.

Question: What is the interpretation of counts for non-zero elements per cell for ATAC-seq data ? Such quantity (or its change) has some biological meaning or purely technical ?

For more information see discussion: https://www.kaggle.com/competitions/open-problems-multimodal/discussion/353629 and notebook: https://www.kaggle.com/code/alexandervc/mmscel-atac-seq-eda02-sums-counts-day-dependence

PS Data are from the Kaggle/NearIPS2022 ongoing competition: https://www.kaggle.com/competitions/open-problems-multimodal