What does refresh_interval ACTUALLY do?

I’m a bit puzzled by the behaviour of refresh_interval in a history-graph card.

I’m trying to minimise the amount of data stored in my database.
My history graph is certainly no more than 400 pixels wide in a browser. Logically, then, more than 400 data points are of no use to me. I have hours_to_show set to 168 (1 week). Since I wish to display no more than 400 points, I set refresh_interval to 1512 (1686060/400). I mis-typed this as “151” and yet my graph still displays a perfectly smooth curve of at least one point per pixel.

Has anyone any idea how this can be?