The only amount it goes over 100,000,000cr is by the amount of paid credits you have.
An awful lot of people have credits remaining from pre-orders and the 25AE - and, more recently, the Power Pack - because paid credits aren't spent until "free" (ones you earn) are exhausted and most people haven't got anywhere near 0cr.
You can check your balance of each with the wallet icon, and it's telling how minor and unobtrusive a feature this is that people don't know the whole free/paid credits thing - while others still insist GT7 is forcing MTX on you.
As a reminder, 100,000,000cr is the cap on credits you earn. There is no cap on credits you buy. The total shown comprises both.