gmail question

[bazookaman]

what determines what gmail saves in the sent or trash? I've gone to retrieve something i KNOW I deleted in my browser and it is not there. Same with sent. I've sent dozens of email from my browser throughout the day and when i got to the sent items, there's maybe 4 or 5. And i cannot seem to make sense of how or why it decided to keep what it does and discard the rest.