Both those bands are special cases in many ways. We need a higher flux level to really get them popping, but there's another mode - referred to as 'E Skip' (written Es for short) during the spring and fall (happens in the summer too, but less so in the winter) where signals can go great distances, but the openings aren't all that long in duration.
It seems that Es is more common during low sunspot activity, then when it's very active, and no one is quite certain as to why. You really have to be johnny on the spot to detect this - which is why hams that are active DXers on these 2 bands sometimes have spare receivers sitting on specific 10 and 6 meter frequencies, waiting for signs of an opening. It's catch as catch can, when it comes to Es. This mode can also be invoked during and just after severe thunderstorms, although it doesn't always happen this way.
Welcome to the mystery world of propagation - there's much we really don't fully understand....73 Mike