Why was it believed in the first place that !a supernovae almost always produce the same amount of light, so if you know how bright it is, you can tell how far away it is?

Most likely because “The star doing the exploding is a white dwarf with a fairly standard mass, so the supernova’s brightness is predictable.” *

and

“For a long time, astronomers figured that since 1a’s were all so similar, there must be just one way to cook them up. A key step in the process of making a 1a is that the white dwarf has to somehow grab enough material to ignite carbon fusion and explode.” *

This assumption was already coming under suspicion however:

“But here’s the thing: Despite their crucial cosmological importance, type 1a supernovae are still very much a mystery. As astronomers study more and more of them, it’s becoming increasingly clear just how non-standard these explosions actually are – and how little we really know about them.”*

This was published in August last year, apparently even before the emergence of discoveries resulting from studying those 1a UV emissions described in the above article.

Chris G

*No Place Like Home: August 28, 2014 Type 1a Supernovae: Why Our Standard Candle Isn’t Really Standard by Nadia Drake
http://phenomena.nationalgeographic.com/2014/08/28/type-1a-supernovas-co...