That just sounds like an attempt to save both of you time (elapsed time, from interview to offer, not time spent) in case your phone interview did do well.

After the phone interview, somebody needs to write it up, a hiring committee needs to look at it, and then they can decide whether to move forward or not. If they give you the challenge after that it's easily another week.

Frankly I am surprised they gave you a full response on Monday after a phone interview Friday, that's actually quite fast given all the steps and people involved.