Why does my Steem bandwidth keep fluctuating?

I've been checking my bandwidth on steemd.com and I'm noticing something weird- it seems to fluctuate constantly. The amount I have used stays constant, of course, but one minute it will say I have 147kb left and the next it says I have 153kb left. I really don't understand what's going on here- even if I'm not causing any activity on my account it will change.

What might be causing my bandwidth to change rapidly?