I'm stubborn and I could be wrong, but I'm still doubting all explanations other than optical so far. For three reasons:
1. There's no indication they are cropping what the display can produce between the 1st and 2nd prototype.
2. With a smaller FOV the prototype could be much smaller, and it isn't.
3. There are clear indications that the eye relief was increased.
#1 and #2 could be for power reason, solved at launch.
BUT... #3 would explains the loss of FOV by itself.
http://arstechnica.com/gadgets/2015/05/01/hololens-still-magical-but-with-the-ugly-taint-of-reality/
The Ars Technica journalist wears glasses, and said the previous prototype had the hololens glass pressed against his prescription glasses. While the new prototype didn't. So it's clear to me they increased the eye relief, and that would absolutely reduce the FOV.
I wonder if there are issues with accessibility laws. Looking at VR products, they would be much smaller if they didn't have to accomodate glasses users, yet all of them did. The ones that didn't were changed to do so. Morpheus had plenty of eye relief at the first public prototype, and it skewed the FOV comparisons with A vs B lenses of DK2. OTOH, I think it was established that Crescent Bay had increased eye relief, and the FOV was reduced. Now both provide a similar perceived FOV at similar eye relief distance. And they are very bulky.
I am probably being stubborn too but the ideal that MS would "profoundly" (as stated by the author of the ars article) change the experience just to accommodate a subset of users seems rather extreme to me. Why not design a unit that accommodates all users but limits the reduction of the FOV to those who must wear spectacles? Google Glass initially didn't really make any real attempt to deal with prescription glasses. And did changing the eye relief of OR or Morpheus dramatically change the FOV?