bISagh'a'? I had no idea there was a specific reproduction rate. I was just making one of my warped-sense-of-humor jokes!
Perhaps "my" (hypothetical) original two tribbles were fed just a few meager grains of quadrotriticale. Thus greatly slowing the reproduction rate. ;-)
BARIS: Gorged? On my grain? Kirk, I am going to hold you responsible. There must be thousands of them.
KIRK: Hundreds of thousands.
SPOCK: One million seven hundred seventy one thousand five hundred sixty one. That's assuming one tribble, multiplying with an average litter of ten, producing a new generation every twelve hours over a period of three days.
KIRK: That's assuming they got here three days ago.
SPOCK: And allowing for the amount of grain consumed and the volume of the storage compartment.
If you do the math, it turns out that factor has no impact. Start with 1 tribble at 0 hours. Then you have
1 + 10 = 11 tribbles at 12 hours
11 + 11 * 10 = 121 at 24 hours
121 + 121 * 10 = 1,331 at 36 hours
1331 + 1331 * 10 = 14,641 at 48 hours
14641 + 14641 * 10 = 161,051 at 60 hours
161051 + 161051 * 10 = 1,771,561 at 72 hours
All the other stuff Spock says is just unimportant verbiage.
Here's a site that summarizes: http://courses.missouristate.edu/AlexanderWait/notes/SPRING%202003/Tribbles.htm