Is there a name for this kind of lying?

…..and are you (sometimes) guilty of it?

I’ll give an example: You have an appointment with your dentist and you fear that he/she will have some critic on your maintenance.
You haven’t flossed as much and as regularly as you promised him/her, half a year ago.
You decide to tell him the following:
“Well, I have to be honest and say that I flossed 8 out of 10 times in reality it was 5 out of 10 times…., no, make that 7 out of 10 times.”
The correction no, make that 7 out of 10 times functions (you assume) as a making-your-statement-more-honest-sounding tactic.

Is there a name for this phenomenon (if it is at all a phenomenon)?
Do you do it? be honest :-)
Why do we do this?