Sorry for the delay in responding and the forum's not hugely active right now. Plus I've been pondering this post for a week or two :-))

Based on my personal experience with paniculatas, this double bloom cycle does not appear to be unique to any particular cultivar. Nor does it appear to be typical. Does it bloom in this manner every year? Do the deer graze the outer stems routinely? And do they do this only in winter or into the early spring as well? Or even in summer as local deer are want to do :-)

Paniculatas bloom on the current season's growth. If you do not routinely prune the plant back each year, then that's why the taller center stems bloom early, while the deer grazed outer stems bloom a bit later. You can experiment with this by cutting back the entire plant in late winter and protecting it from any deer predation. I'll bet the predominate bloom period will be simultaneous but you will still usually get a lttle bit of later bloom from later emerging new growth.

If you've had this shrub for 13 years, I'd guess one of the older, traditional cultivars like Tardiva or Unique (which can reach 14').

No real science behind this....just personal experience with these plants :-)