Hi HB, interesting but could it be that when they first start flowering, the days are quite long giving more growing hours for these stems to develope ? Also, as you get more and more flowers (as the season wears on) the 'energy' within the plant, has to be divided between many more stems ??
I'm sure that if you removed some of the many flower stems being produced later in the season, the ones left would also be bigger and longer ? A bit like the principle of disbudding chrysanthemums to produce larger flowers ??
Lannerman