Yes, I don't know why it is doing that. You should be able to take it out further, to the critical duration, to get your max required storage.
Consider the two calculations I ran with identical basin parameters: A = 10 ac, C = 0.75, Tc = 20 min, Qtarget = 1.0 cfs below:
I ran one in Hydraflow Hydragraphs shown first below, the second was in Hydraflow Express, shown below that.
Hydraflow Express has a couple of display issues going on also: