I suspect that most of the price variation is related to dividend/interest accumulation in the fund and timing. In other words, if the fund is prices at $10.67 and pays a $0.20 dividend and drops to $10.47 on it's ex-dividend date you have not actually lost anything or seen a decline. It is also completely predictable. It's not a random variation in teh normal sense.
I never thought of that, but it appears to be pretty accurate. I checked the prices on dates around distributions and it seems to drop in accordance with the amount of the distribution.