Most people are kiasu, and so they want the best bang for their buck. Ultimately, if your account value is $0 on the day of account opening, your counter does not tick up at the end of the month; which is perhaps what people are most concerned about. Losing that 1 counter, or 0.1% interest as a result.
The calculation that the maximum interest at stake is only $1.20 or $12 is flawed, because bonus interest is not subject to any cap; only base interest. Hence, if I am able to fund my account with $100million, the interest at stake is $8.3k. Ultimately, it's about 0.1% interest; even if I pushed the funds in 5 months after opening, 1 counter late means that interest is lost.
Those who are concerned about FAST'ing their money in ASAP are also misguided. Simply deposit $1 when the account is opened, and your counter will tick up at the end of the month.
That said, bonus interest is 0.1% p.a per counter; that means, if you opened your account with 100m on the 1st, $8.3k of interest is at stake; but if you opened the account on the 15th, only $4.15k of interest is due, since it's just there for half a month.
Ultimately, once the account is opened, the best time to fund it is the first of the month; but, sometimes (and it's happened to me), I FAST in on the first, the money shows up, but a few days later, the transaction was timestamped on the 2nd (even though the first was a weekday). Since then, I've made it a point to top up the account at the end of the month instead.
Finally, never keep this account at $0. Even with $1, the counter ticks up. Combined with (a) having multiple accounts, and (b) Citi uses a TRB system, instead of a 'per account' minimum balance, it's not difficult to figure out how to setup this account, with the ability to keep 12 counters on your balance, even with the ability to withdraw... how often you can withdraw 'without penalty' however depends on how many people you trust enough with all your money...