My interpretation: 1) they always charged for SSL overhead; 2) they introduced a bug in late 2016 that underestimated SSL overhead; 3) that reduced charges for SSL overhead, and also broke flagging of Spark and Flame plans with too much SSL overhead; 4) they just fixed the bug, restoring normal estimates of SSL overhead; and 5) that restored accurate charges for SSL overhead, and also restored flagging of Spark and Flame plans with too much SSL overhead.
I'm guessing that OP's SSL overhead spiked after the measurement bug was introduced. Maybe also Firebase wasn't enforcing their limits before introducing the measurement bug, because they weren't confident about the data.
> The Firebase Realtime Database charges for SSL overhead on all requests ... We’ve always had a policy of charging this way.
So it's always been charged.
> Unfortunately, we introduced a bug late last year that began undercharging for SSL
Except it hasn't always been charged?
> We recently started actually enforcing overages on legacy plans and our current fixed-price ($25/mo Flame) plan.
In fact, if you're on a Spark or Flame plan, it's never been charged?
I feel like this response is straight from Office Space or something. "So we just went ahead and fixed the glitch."