A weird behavior of SSD

I just love my new MBP that came with Apple SSD in it. However, I have only 128GB SSD so I don't want to fill it up too quickly. Today, I deleted tons of old emails in the Mail app since I remember that doing so can free up some space in HDD in my iMac. Surprisingly, I found that after I deleted those emails and emptied the trash, the used space of SSD went from 40.45GB up to 40.51GB . That was not the end of the story because 10 minutes after that the used space came down to 40.48GB . I monitored the used space of SSD by istat pro, if that matters.

Any idea of what was going on in SSD?