I think you have just unfortunately have 2 products on the smallest and largest spectrum of their manufacturing tolerances. So the Challenge tires are most probably the smallest diameter/circumference within tolerances, and the wheel is at the largest end, which means they don't fit. And no it's unlikely the tires will stretch that much, otherwise the danger of the tires popping off the rim is a real possibility.
I just got the Challenge Bianca Strada Pro TLR, and it mounted ok, but like yours, it was tight in the center channel, but I use it tubeless, so it took a bit of work but I finally managed to get it to fill and hold on both wheels. So