I noticed line 42 of hardware/cores/arduino/WMath.cpp contains:
return random() % howbig;
Due to the modulo bias this will result in a small change from the uniform distribution expected given a suitable PRNG.
The correct way of doing this could be:
long val, limit;
limit = RAND_MAX - RAND_MAX % howbig;
while ((val = random() >= limit);
The practical importance of this is low. I don't imagine anyone to particully care, but I thought I'd mention it. It may have been a design choice, though in this particular case people often are simply not aware of the slight bias they introduce.