Bought the game yesterday, and received 500 cryptic points. I have played up to Lt. 7 at the moment with only one character. So, I just noticed that now I have 900 cryptic points. Where did the other 400 come from?

The stipend that gold members get started last month. As long as you remain subscribed, you get 400 points per month. That's probably where it came from.

Yep, your subscribers stipend 400 a month from the date of your subscription.