Just to answer the question:
We're on the Salty Shores update, but the devs did confirm the calculation logic. Keep in mind that the numbers have changed (due to feedback from players that too little event currency was gained per game), but the principle is still the same.
- Players receive a minimum of 10 shells for a completed game (i.e. full 5 minutes - no overtime). This is actually a randomized number, but it is never lower than 10. From experience, it seems to be 10 to 15.
- If a game ends sooner (forfait, early leavers), the amount of shells earned scales in relation to the % of match time played. This is to prevent obvious quit/join abuse to get a lot of shells in a short time.
- Overtime scales up the rewarded shells. Based on experience, it seems like it's a linear progression. For 5 minutes of overtime (= 200% match time in total), I always received 20-30 shells (= 200% of normal payout)
The devs mentioned that they roughly doubled the event currency gained per game because players complained that they weren't getting rewards fast enough.
I read a (verified) dev's comment on Reddit, but I can't find it at the moment.
This means that your observation is coincidence. However, the amount of currency gained (if they did double it, we would initially get 5-8 flowers for a completed game) sounds like a reasonable total for goals+shots+saves as well.