Peculiar reflection albedo from International Space Station

I was viewing the night sky, looking to the north, and noticed a quite bright light, which was moving across the sky, from my left to right.
 At first, I thought the light was the flare from an Iridium flare, many of which I have seen before tonight.  That was my first impression, given that the illumination from the object appeared to flare up, and then begin to subside, much the way an Iridium satellite is seen to do.
 Then, however, perhaps 2-3 seconds after the flare subsided, the reflection flared up a second time, such that the second flare was brighter than the first.  This surprised me, since I had never seen an Iridium satellite flare more than once.
 Not knowing what the object was, I recorded the time, and then immediately checked the www.Heavens-Above.com website for Iridium flares, and discovered that it had, indeed, been the International Space Station that I had observed.
 Perhaps there are more items attached to the ISS framework that create a “double flare” effect, but in the dozens, or hundreds, of times that I have witnessed an overflight of the ISS, I have never observed that effect.  In fact, I have never witnessed the reflection albedo from the ISS to fluctuate, at all, until it has flown into the Earth’s shadow.
 The sky was crystal clear, at the time of the sighting, so variable light overcast probably was not the cause of the variation in illumination.
 I will observe the ISS on subsequent nights, in order to determine whether this is a normal phenomenon.