Ya, that's not as odd as you would think. That's kind of been the problem with this parser. The Office Open XML "standards" can be interpreted in multiple ways, even inside Microsoft, so they're generating the XML differently between all the different tools, but my code started based on how Excel generates it, and I've been trying to expand over the years to handle more and more variations.
I think the situation here is that SSRS is using a streaming format whereas Excel writes it out from memory, and the streaming format takes shortcuts.
Bruce Dunwiddie