Sorry about that!
Special Event currency often provides a bonus on its own. Some event boosts are purchased using this Event Currency. There may be times where spending your event currency for a boost actually results in a net loss. This is because you're losing the bonus from the Event Currency itself in exchange for an Event Bonus.
Secondly, please double check if the bonus in question is multiplicative or additive.
100 +10% +10% 40% = 140
100 +10% ( =110) + 10% ( =121) +20% = 145
With larger numbers, the difference becomes even greater.
If you need help troubleshooting or are experiencing an error with this, please contact support.