Cannot find reason why differential backups size reduced Sql Backup job summary:

Hi, I have a backup job that take full backup first day of the month and take differential backup every 2 hours between 9am and 5pm everyday.

Problem:
While testing disaster recovery plan I found that after few differential backups size of differential backup reduced and when I include reduced sized differential backup(red square) I get this error: in restore    

Any idea why size of differential backup reduced? and how I can restore this differential backup?

Please help and thanks in advance for reading and answering this question.

Update
As mentioned job is taking full backup start of month there is no full backup between differential backup size of differential backup is increasing till position 91 However, as you see in image no full backup is taken before position 92.
The most likely cause by far is that a full backup was taken out of sequence (between that differential and the previous full backup) - that makes the differential smaller as there are less differences to include between it and this extra backup than between it and the expected previous backup.

If this is the case then to be able to restore the unexpectedly small differential you need to find a copy of the extra backup.

This is usually because someone has taken a copy for use elsewhere, perhaps for testing a support issue or new release. IT is important that everyone who has access to be able to take backups knows that they need to be careful of this because it breaks your DR plans and so forth.

There is a safe way to take an extra backup: use the COPY_ONLY option. This takes the backup as normal but does not interfere with your main full/diff/log backup sequence.