New Flyer does have terrible build quality, or at least it does now, which I find most transit vehicle manufacturers do at this time, but I digress.
The differences as far as the original Low Floor and the Xcelsior are quite stark looking in to it..
The LF had began development in 1988, and entered production three years later in 1991. Arguably the LF is derivative of the standard floor equivalents of the time, which are now unofficially referred to as HF's. As such the structural components are comprised of that made the HF's durable were carried over to the LF's. Further than that the model pioneered the semi low floor design which is predominately found in North American transit buses today.
The Xcelsior in contrast was a platform that began development in 2008 and didn't take off in the market for a good two years, until 2010. The intent with the Xcelsior was to revolutionize the industry by offering a vehicle that would lower costs and deliver performance as well as aesthetics at a reasonable rate. To achieve that New Flyer resorted to using different materials to achieve a lightweight structure which would give a greater fuel economy and reduce build costs. When possible composite and other lightweight components were used in place of stainless steel, etc, etc.
So in summary it's most likely a weight differential, along with an increased backlog which may rush production.