Why tapes write half of the data to tape and jumps to other tape to write remaining data?

Hi Everyone,

I created two tapes of 5TiB of each in AWS.

I have a backup job which backups up 1.25 TB of data.

When I start the backup job, it is selecting the tape and writing the half of the data on 1st Tape(413 Gb) and jumping to other tape to write the remaining data(856 GB) on 2nd tape. I can't able to utilize the full space of the tapes.

Is this tape behavior from AWS Side OR Net Vault Backup?

Thank you in advance.