English/BSC and ISO are similar but not the same. Both use a 60 degree included angle but ISO has a larger nominal diameter of 1.375" versus 1.370" for English. The roots and crests of ISO threads are flat while English/BSC has rounded crests and roots. In this case, ISO is intermediate threading that mates well with both Italian and English/BSC.

When the ISO standards were created, the committee looked at all the conventions and made their decisions based on which would have the least impact on the industry. Since BSC threading had been adopted by the Asians, who were producing the majority of the world's bicycles at the time, most but not all of the ISO bicycle thread standards use, or are compatible with, English/BSC. For instance, the selected standard for dropout hangers is the metric 10mm x 1mm thread, as most of the world followed the French practice. In some cases, there are both primary and secondary standards (i.e. 9/16" and 1/2" pedal threads).


Another thing to clear up is that French is not the "oddball" threading. That honor belongs to the Italians, courtesy of Campagnolo, who combined a metric nominal diameter with an imperial thread count (i.e Italian freewheel threads are 35mm x 24 TPI). Every public school student knows that you do not mix imperial and metric measurements, but this apparently escaped Tullio and his countrymen. And of course the Italians continue to perpetuate the non-standardization of bicycle parts by continuing to build new bicycles with Italian threading.