As the title asks, I'm drawing a blank why the original 100w T17 went to 90w. It may have been explained at some point but I can't remember. Anyway, my understanding is that this change was made fairly early on in either the late 40's or during the 50's? This was way before the concept of 'energy saving' fluorescents were the rage so I'm thinking there is a technical reason.

Also to mention, as other members like DieselNut, my 90w lamps can be stubborn to start...sometimes I swap out starters and that helps. The ballast has a relatively low OCV (~160v) which seems a bit low for such a large lamp. The 90w lamp has a slightly lower operating voltage than the 100w but I wonder if the 100w were as stubborn as some of the 90w.

It would be interesting to see the design concepts of these lamps and what flaws they may have been trying to deal with.

Even in that era the energy saving was one of the major development forces - the electricity was rather expensive...
And at the same time it was time of rather rapid lamp improvements (fluorescents still in their infancy), so I would see as very likely the motivation was really to offer lower energy consumption (that also means thermal load) with the same light output...