More than 400 points?

You assumed correctly. ;)

Yes, that's exactly why.

You also can get more than 400 if you remove a giveaway.

To be exact there are two most possible options:

1. Someone deleted giveaway and you got points back
2. You won a giveaway, so you got kicked from any other giveaway of the same game - and you got points back.

you can also hoard points. to do so enter all the giveaways that last as long as possible until youre close to 0 points, then when the points reaccumulate, enter more giveaways that last a long time. then before all those giveaways end, exit those giveaways so you can hoard up to 1000+ more points when theres some bundle and everyones giving away that same game(s) that you wanna enter :D