Though I have no presentable basis for it, it's not a glitch/bug. The same way your Kurito prize varies, the number/score that determines your wait time from Kurito isn't fixed and it fluctuates, from what I've seen only above what your original score presents but never below. The higher or more far out the fluctuation is, the longer the wait time and though it doesn't give you more prizes or higher monetary value ones, by putting you in a longer wait time the prize comes from that wait time tier as well.
Thus occasionally offering a 0:50 donation which usually leads to a 0:50 wait time may instead fluctuate and trigger a 1:30 wait time, sometimes even a 2:00. It's not going to be guaranteed to give you a better prize, just that the prize will instead be coming from the 1:30/2:00 wait time prize list instead (which may be better, may be worse depending on how you value it personally).
The same goes for other donation types. 1:30 donations may result in 2:30 wait times (I got the SR Lim this way) and I'm fairly sure 2:00/2:30 donations can push for 3:00/4:00 wait times depending on luck/circumstance.