I realize bonds go up and down based on inflation and perceived threats. What I don't really get is why bond ETFs and bond funds which hold only the most conservative north American government bonds will go up and down from day to day when those factors haven't changed.

The bolded sentence is wrong. That's why you aren't getting it.

As Ken Fisher continually says (as well as others), stock & bond prices are set by supply and demand. Period.

The things you mentioned are (some of the) factors that buyers & sellers take into account when they decide to sell (supply) or buy (demand).

(BTW, this is also why WMT price hasn't changed much in the last 10 years, despite the booming increase in valuations. Valuations don't make the price go up, only supply and demand make the price go up.)