That number seems high, but it doesn't explain how I actually got 4400 gold in 13 minutes.
And minion gold increases by 1 every 5 minutes. In this case the gold earned would have been capped at 24g and 18g... so even 20g is a low estimate since a majority of the 13 minutes is less then that. Besides, I don't know about you, but I normally get 100 CS by 13 minutes, not 89. Especially since I was playing Morgana. I just wanted a lower CS number so I missed the first wave to simulate super heavy harass. So in practical terms, this is actually a really low estimate.
The end result is this. Somehow, with all the gold/10, I managed to get 4400 gold by 13 minutes. How did this happen? Where did the gold come from? You can try it yourself. Gp/10 quints are super cheap. Even if I increased the average gold to 25 on minion kill, it's still only 2225 gold. That's barely 1/2 of the required amount. And if we go by your numbers, that's still 500 gold unaccounted for. So unless I managed to secretly kill 4 dragons as part of the 89 cs, something is going on that we can't see.
This is what I mean by "I don't know what to make of this".
I also took the gold/10 into two normal games as jungle Skarner and jungle Shyvana. In both cases I actually managed to get Wriggles by 6 minutes even though I didn't kill anyone yet. I didn't buy another gp/10 item in these though.
Tl;dr: gp/10 might be bugged somehow and is giving more gold than it actually is saying.