Lots of time is rather colloquial, maybe that's why. I guess they can't enter every possible answer into the program, which means there are plenty of things that could be accepted that aren't.
Good attitude! It really is amazing. I took Spanish for a few years awhile back, before Duolingo was available, and I paid a lot of money for several years of "continuing education" classes. I honestly feel like I've learned way more Italian from Duolingo in the 3 months I've been on it, than I learned from 2 years of Spanish classes.