Once an email is sent, new information is collected as to which email address is now valid or not valid anymore. The email invalid flag is only as current as the last email send. As long as your hard bounce rate is under 3% then you are fine when it comes to your deliverability rate.
I hope this information helps!
There are many reasons why emails are bounced and there are many posts in community here explaining these, for example Email Bounce Codes, Making sense of Marketo email bounce categories, How do you make your email deliverability unstoppable? etc.
Did you investigate what kind of bounce types are you getting? And what deliverability rate are you talking about?