In the digital age the destruction of data is a really complex process.
A lawyer is going to multiple digital copies at different locations.
He will probably have both laptop and desktop computers synced together.
Will have off-site backups and possibly on-site backups.
If it is really important, he might have it on thumb drive or DVD hard copy it a safe.
So, it might be very easy to forget to destroy one or two copies.
Then there is always the possibility that someone hacked your computer and stole the data.
These things happen
Might have given the client a copy, too. “Hey, you are under orders to destroy your copy. Honor system!”