Not sure why you got that result. This was the top result I obtained when I asked Google "oxidation number for N2O5":
The link leads to the post with the first answer strongly upvoted at the very top and in this case the Google algorithms served me that content in the main results page (although I did not see an infobox).
Your mileage might vary, horse's mouth, caveat emptor, etc
The problem is therefore not fully reproducible. Something happening on Google's end or in the way SE feeds pages to Google is probably cause for variation between what different users see in the search results.
Following some inquiries, it seems in general the mixed quality of the IA-driven page summaries provided by Google is a known issue. This seems out of our control.
However, you can still do a few things: Google provides a way to submit feedback in the infoboxes and search result pages. This might help them improve their IA engines. Use that if you want.
In addition, the engineers at SE are busy figuring out how to help Google's crawlers. That's their job. You can go to main meta and ask for a progress result if you want to pressure them along.
That of course does not solve the immediate problem of infoboxes feeding you dodgy info. Until further notice the policy might have to be caveat emptor. Open that horse's mouth and take a close look before you use that info! And be glad that at least the bad info was free.