Nice, I thought the birthday reward is fixed amount based on our VIP level but your story proves that what I thought is wrong.
Probably you get that much because of your recent crazy wager achievement, something like when you wager about half a million dollar in a week last month IIRC.
In my own experience, I've received the birthday bonus 2-3x and the amount were all the same ($25).
I guess you are getting closed to Diamond now, are you more than 50% now or less to Diamond?