Fluctuation in allocated space

I’m really concerned about the allocation space fluctuations in nextcloud.
For example:
I have allocated 36 GB to a user in ncloud, every day the user uploads two 2.3 GB files, one in the morning and one in the evening. It’s done every day except for public holidays and on Saturdays. Only previous 3 day’s data is stored and any file/data other than the 3 day period is removed/deleted from the ncloud as well as from the ncloud trash.
What is really concerning about this issue is that, the allocated space (36GB) keeps fluctuating and sometimes shows 25 GB or even lower, it’s really confusing how the allocated space keeps fluctuating while uploading files or sometimes even when the uploads have completed.
Isn’t the allocated space supposed to be constant and the used storage be fluctuating?
Kindly help me out on this.

Thanks in advance,
Hoping to hear from you soon.